Xen devel - Mar 2011 - xc: error: xc_machphys_mfn_list: 83 != 129 when suspending 32GB PV DomU

Hi,

I have a 256GB host and run a 32GB 64-bit PV domain (SLES 11) on it.
When I try and suspend the domain, xc barfs with:

 xc: error: xc_machphys_mfn_list: 83 != 129: Internal error
 xc: error: xc_get_m2p_mfns (0 = Success): Internal error
 xc: error: Failed to map live M2P table (0 = Success): Internal error

At first, since dom0 is 32 bit, I suspected the compat layer. However
the hypercall in xen/arch/x86/x86_64/compat/mm.c compat_arch_memory_op()
seems to agree with the numbers:

 (XEN) compat_arch_memory_op returned 0 (nr_extents = 83, max_extents = 129)
>From this I conclude that everything is working OK at the hypercall
layer. However, looking at the code in compat_arch_memory_op() it
appears that the code is failing due to some arcane limits of the compat
subsystem. The following code to establish the variable
''limit'' is
causing the loop to exit early:

    limit = (unsigned long)(compat_machine_to_phys_mapping +
        min_t(unsigned long, max_page,
              MACH2PHYS_COMPAT_NR_ENTRIES(current->domain)));
    if ( limit > RDWR_COMPAT_MPT_VIRT_END )
         limit = RDWR_COMPAT_MPT_VIRT_END;
    for ( i = 0, v = RDWR_COMPAT_MPT_VIRT_START, last_mfn = 0;
         (i != xmml.max_extents) && (v < limit);
          i++, v += 1 << L2_PAGETABLE_SHIFT )
    {
        /* do stuff */
    }
    xmml.nr_extents = i;

Further debugging reveals the variables are set as such:
 (XEN) compat_machine_to_phys_mapping = 18446606377058041856
 (XEN) max_page = 67272704
 (XEN) MACH2PHYS_COMPAT_NR_ENTRIES(current->domain) = 43515904
 (XEN) RDWR_COMPAT_MPT_VIRT_START = 18446606377058041856
 (XEN) RDWR_COMPAT_MPT_VIRT_END = 18446606378131783680
 (XEN) limit = 18446606377232105472, (1 << L2_PAGETABLE_SHIFT) = 2097152

Could it be that the compat mach-to-phys conversion table size of 1GB is
too small? Or that there exists some other arbitrary limit on the size
of domains that can be suspended [when using a 32bit dom0] ?

Gianni


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On 11/03/2011 18:52, "Gianni Tedesco"
<gianni.tedesco@citrix.com> wrote:
> Further debugging reveals the variables are set as such:
>  (XEN) compat_machine_to_phys_mapping = 18446606377058041856
>  (XEN) max_page = 67272704
>  (XEN) MACH2PHYS_COMPAT_NR_ENTRIES(current->domain) = 43515904
>  (XEN) RDWR_COMPAT_MPT_VIRT_START = 18446606377058041856
>  (XEN) RDWR_COMPAT_MPT_VIRT_END = 18446606378131783680
>  (XEN) limit = 18446606377232105472, (1 << L2_PAGETABLE_SHIFT) =
2097152
> 
> Could it be that the compat mach-to-phys conversion table size of 1GB is
> too small?
It is insufficient to cover all of the system''s memory. The reason for
the
limit is that a 1GB M2P table is all that is reasonable to map into a 32-bit
domain''s address space while still leaving space for the
guest''s own
mappings.

We could make the compat m2p bigger solely for mapping by dom0 when doing
save/restore? However, you''d likely still soon hit the mmap limit for
the
save/restore process in dom0. It might extend the lifetime of your 32-bit
dom0 for a bit longer however. Long enough for the lightweight HVM container
for PV guests work to get checked in and make our PV 64-bit guest
performance better.

 -- Keir
> Or that there exists some other arbitrary limit on the size
> of domains that can be suspended [when using a 32bit dom0] ?


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On Fri, 2011-03-11 at 19:21 +0000, Keir Fraser wrote:
> On 11/03/2011 18:52, "Gianni Tedesco"
<gianni.tedesco@citrix.com> wrote:
> 
> > Further debugging reveals the variables are set as such:
> >  (XEN) compat_machine_to_phys_mapping = 18446606377058041856
> >  (XEN) max_page = 67272704
> >  (XEN) MACH2PHYS_COMPAT_NR_ENTRIES(current->domain) = 43515904
> >  (XEN) RDWR_COMPAT_MPT_VIRT_START = 18446606377058041856
> >  (XEN) RDWR_COMPAT_MPT_VIRT_END = 18446606378131783680
> >  (XEN) limit = 18446606377232105472, (1 << L2_PAGETABLE_SHIFT) =
2097152
> > 
> > Could it be that the compat mach-to-phys conversion table size of 1GB
is
> > too small?
> 
> It is insufficient to cover all of the system''s memory. The reason
for the
> limit is that a 1GB M2P table is all that is reasonable to map into a
32-bit
> domain''s address space while still leaving space for the
guest''s own
> mappings.
The compat M2P actually mapped into the guest isn''t 1GB, 1GB would be
the entire kernel mapping with no room for anything else. Also 1GB of
M2P is enough to cover 1TB of host memory so I don''t think
it''s too
small at the moment. Is the limit here not MACH2PHYS_COMPAT_NR_ENTRIES?
(in the above limit == compat_machine_to_phys_mapping + ~160M)

IIRC the size of the M2P which is mapped into a PAE guest is normally
capped at ~160M (the total size of the hypervisor hole for a PAE guest
running on a PAE hypervisor). 160M is enough M2P for 160G of host
address space which would explain why this is seen on a 256GB host but
not a 128GB one.

The limit on the size of the M2P is adjustable, in particular for dom0 I
think it would be reasonable to allow it to expand to, e.g. 256M,
without too much cause for concern.

Obviously this hole eats into the 1GB kernel mapping so you don''t want
it to grow too much bigger and long run something better would be needed
but this would probably allow you to support 256GB without too much
trouble in the short term, other than slightly reducing the amount of
lowmem the system sees (which might be an issue if you''ve chosen
dom0_mem on that basis...)

The lower limit is set by the kernel in its XEN_ELFNOTE_HV_START_LOW ELF
note (set in arch/x86/kernel/head_32-xen.S), which is picked up in
xen/arch/x86/build_domain.c:construct_dom0(). NB: This might be the
first time this functionality has been used in anger to increase the M2P
space (I think it is actively used to shrink it on hosts with <160G).

Another alternative, which would allow large hosts without needing to
expand the dom0 M2P, would be to provide interfaces that allow the tools
to map specific portions of the host M2P so the tools can build
themselves a mapcache style thing. The M2P space which needs to be
accessed to perform a migration of an individual guest is likely going
to be smaller than the total host RAM so even using 256M-512M of guest
user-mode address space (allowing for 256GB-512GB of host address space)
would likely allow you to map the bits you need without excessive churn
(aka performance hit) in the mapping. A given userspace process has 3G
of address space to play with so it can take the hit of increasing the
M2P mapcache size far easier than the kernel can. Hrm, maybe you don''t
even need a map cache thing -- just a way to allow a userspace process
to map more M2P than the kernel can... (which might be as simple as
removing the limit clamp based on MACH2PHYS_COMPAT_NR_ENTRIES in the
compat layer?)

Ian.



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This permits suspend/resume to work with 32bit dom0/tools. AFAICT the
limit to MACH2PHYS_COMPAT_NR_ENTRIES is redundant since that refers to a
limit in kernel mappings under 32bit hypervisors, not userspace where
there may be gigabytes of useful virtual space for this.

Suggested-by: Ian Campbell <Ian.Campbell@eu.citrix.com>
Signed-off-by: Gianni Tedesco <gianni.tedesco@citrix.com>

diff -r cf558cb8b92b xen/arch/x86/x86_64/compat/mm.c
--- a/xen/arch/x86/x86_64/compat/mm.c	Mon Mar 07 17:52:44 2011 +0000
+++ b/xen/arch/x86/x86_64/compat/mm.c	Mon Mar 14 14:58:04 2011 +0000
@@ -162,8 +162,7 @@ int compat_arch_memory_op(int op, XEN_GU
             return -EFAULT;
 
         limit = (unsigned long)(compat_machine_to_phys_mapping +
-            min_t(unsigned long, max_page,
-                  MACH2PHYS_COMPAT_NR_ENTRIES(current->domain)));
+                    (unsigned long)max_page);
         if ( limit > RDWR_COMPAT_MPT_VIRT_END )
             limit = RDWR_COMPAT_MPT_VIRT_END;
         for ( i = 0, v = RDWR_COMPAT_MPT_VIRT_START, last_mfn = 0;



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Cc''ing Jan. I''d appreciate his Ack as this is his code
originally. I don''t
think we''re missing anything subtle though.

 -- Keir

On 14/03/2011 15:05, "Gianni Tedesco"
<gianni.tedesco@citrix.com> wrote:
> This permits suspend/resume to work with 32bit dom0/tools. AFAICT the
> limit to MACH2PHYS_COMPAT_NR_ENTRIES is redundant since that refers to a
> limit in kernel mappings under 32bit hypervisors, not userspace where
> there may be gigabytes of useful virtual space for this.
> 
> Suggested-by: Ian Campbell <Ian.Campbell@eu.citrix.com>
> Signed-off-by: Gianni Tedesco <gianni.tedesco@citrix.com>
> 
> diff -r cf558cb8b92b xen/arch/x86/x86_64/compat/mm.c
> --- a/xen/arch/x86/x86_64/compat/mm.c Mon Mar 07 17:52:44 2011 +0000
> +++ b/xen/arch/x86/x86_64/compat/mm.c Mon Mar 14 14:58:04 2011 +0000
> @@ -162,8 +162,7 @@ int compat_arch_memory_op(int op, XEN_GU
>              return -EFAULT;
>  
>          limit = (unsigned long)(compat_machine_to_phys_mapping +
> -            min_t(unsigned long, max_page,
> -                  MACH2PHYS_COMPAT_NR_ENTRIES(current->domain)));
> +                    (unsigned long)max_page);
>          if ( limit > RDWR_COMPAT_MPT_VIRT_END )
>              limit = RDWR_COMPAT_MPT_VIRT_END;
>          for ( i = 0, v = RDWR_COMPAT_MPT_VIRT_START, last_mfn = 0;
> 
> 


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On Mon, 2011-03-14 at 15:05 +0000, Gianni Tedesco wrote:
> This permits suspend/resume to work with 32bit dom0/tools. AFAICT the
> limit to MACH2PHYS_COMPAT_NR_ENTRIES is redundant since that refers to a
> limit in kernel mappings under 32bit hypervisors,
32 bit guest on 64 bit h/v, but yes, it refers only to the size of the
mapping of the compat M2P which the hypervisor provides for the guest in
the "hypervisor hole".
> not userspace where
> there may be gigabytes of useful virtual space for this.
Agreed, and in any case if the guest/tools wants to ask for more
mappings than it can cope with, that''s its own problem...
> Suggested-by: Ian Campbell <Ian.Campbell@eu.citrix.com>
> Signed-off-by: Gianni Tedesco <gianni.tedesco@citrix.com>
> 
> diff -r cf558cb8b92b xen/arch/x86/x86_64/compat/mm.c
> --- a/xen/arch/x86/x86_64/compat/mm.c	Mon Mar 07 17:52:44 2011 +0000
> +++ b/xen/arch/x86/x86_64/compat/mm.c	Mon Mar 14 14:58:04 2011 +0000
> @@ -162,8 +162,7 @@ int compat_arch_memory_op(int op, XEN_GU
>              return -EFAULT;
>  
>          limit = (unsigned long)(compat_machine_to_phys_mapping +
> -            min_t(unsigned long, max_page,
> -                  MACH2PHYS_COMPAT_NR_ENTRIES(current->domain)));
> +                    (unsigned long)max_page);
max_page is already unsigned long, so only the overall expression needs
casting (since compat_machine_to_phys_mapping is an int *), right?

Ian.



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On Mon, 2011-03-14 at 15:11 +0000, Ian Campbell wrote:
> On Mon, 2011-03-14 at 15:05 +0000, Gianni Tedesco wrote:
> > This permits suspend/resume to work with 32bit dom0/tools. AFAICT the
> > limit to MACH2PHYS_COMPAT_NR_ENTRIES is redundant since that refers to
a
> > limit in kernel mappings under 32bit hypervisors,
> 
> 32 bit guest on 64 bit h/v, but yes, it refers only to the size of the
> mapping of the compat M2P which the hypervisor provides for the guest in
> the "hypervisor hole".
Ah, thanks for clarification.
> > not userspace where
> > there may be gigabytes of useful virtual space for this.
> 
> Agreed, and in any case if the guest/tools wants to ask for more
> mappings than it can cope with, that''s its own problem...
> 
> > Suggested-by: Ian Campbell <Ian.Campbell@eu.citrix.com>
> > Signed-off-by: Gianni Tedesco <gianni.tedesco@citrix.com>
> > 
> > diff -r cf558cb8b92b xen/arch/x86/x86_64/compat/mm.c
> > --- a/xen/arch/x86/x86_64/compat/mm.c	Mon Mar 07 17:52:44 2011 +0000
> > +++ b/xen/arch/x86/x86_64/compat/mm.c	Mon Mar 14 14:58:04 2011 +0000
> > @@ -162,8 +162,7 @@ int compat_arch_memory_op(int op, XEN_GU
> >              return -EFAULT;
> >  
> >          limit = (unsigned long)(compat_machine_to_phys_mapping +
> > -            min_t(unsigned long, max_page,
> > -                  MACH2PHYS_COMPAT_NR_ENTRIES(current->domain)));
> > +                    (unsigned long)max_page);
> 
> max_page is already unsigned long, so only the overall expression needs
> casting (since compat_machine_to_phys_mapping is an int *), right?
Right you are:

--

This permits suspend/resume to work with 32bit dom0/tools. AFAICT the
limit to MACH2PHYS_COMPAT_NR_ENTRIES is redundant since that refers to a
limit in 32bit guest compat mappings under 64bit hypervisors, not
userspace where there may be gigabytes of useful virtual space available
for this.

Suggested-by: Ian Campbell <Ian.Campbell@eu.citrix.com>
Signed-off-by: Gianni Tedesco <gianni.tedesco@citrix.com>

diff -r 8b5cbccbc654 xen/arch/x86/x86_64/compat/mm.c
--- a/xen/arch/x86/x86_64/compat/mm.c	Mon Mar 14 14:59:27 2011 +0000
+++ b/xen/arch/x86/x86_64/compat/mm.c	Mon Mar 14 15:17:59 2011 +0000
@@ -161,9 +161,7 @@ int compat_arch_memory_op(int op, XEN_GU
         if ( copy_from_guest(&xmml, arg, 1) )
             return -EFAULT;
 
-        limit = (unsigned long)(compat_machine_to_phys_mapping +
-            min_t(unsigned long, max_page,
-                  MACH2PHYS_COMPAT_NR_ENTRIES(current->domain)));
+        limit = (unsigned long)(compat_machine_to_phys_mapping + max_page);
         if ( limit > RDWR_COMPAT_MPT_VIRT_END )
             limit = RDWR_COMPAT_MPT_VIRT_END;
         for ( i = 0, v = RDWR_COMPAT_MPT_VIRT_START, last_mfn = 0;




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>>> On 14.03.11 at 16:19, Gianni Tedesco
<gianni.tedesco@citrix.com> wrote:
> 
> This permits suspend/resume to work with 32bit dom0/tools. AFAICT the
> limit to MACH2PHYS_COMPAT_NR_ENTRIES is redundant since that refers to a
> limit in 32bit guest compat mappings under 64bit hypervisors, not
> userspace where there may be gigabytes of useful virtual space available
> for this.
> 
> Suggested-by: Ian Campbell <Ian.Campbell@eu.citrix.com>
> Signed-off-by: Gianni Tedesco <gianni.tedesco@citrix.com>
> 
> diff -r 8b5cbccbc654 xen/arch/x86/x86_64/compat/mm.c
> --- a/xen/arch/x86/x86_64/compat/mm.c	Mon Mar 14 14:59:27 2011 +0000
> +++ b/xen/arch/x86/x86_64/compat/mm.c	Mon Mar 14 15:17:59 2011 +0000
> @@ -161,9 +161,7 @@ int compat_arch_memory_op(int op, XEN_GU
>          if ( copy_from_guest(&xmml, arg, 1) )
>              return -EFAULT;
>  
> -        limit = (unsigned long)(compat_machine_to_phys_mapping +
> -            min_t(unsigned long, max_page,
> -                  MACH2PHYS_COMPAT_NR_ENTRIES(current->domain)));
> +        limit = (unsigned long)(compat_machine_to_phys_mapping +
max_page);
While doing this shouldn''t hurt (except slightly for performance of
the hypercall), I don''t see why it''s useful: For slots past
MACH2PHYS_COMPAT_NR_ENTRIES(current->domain) you
wouldn''t read non-null page table entries anyway (up to
RDWR_COMPAT_MPT_VIRT_END), so I don''t see why the tools
couldn''t equally well do with what we have currently (after all
they get told how many slots were filled).

Jan
>          if ( limit > RDWR_COMPAT_MPT_VIRT_END )
>              limit = RDWR_COMPAT_MPT_VIRT_END;
>          for ( i = 0, v = RDWR_COMPAT_MPT_VIRT_START, last_mfn = 0;


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On Mon, 2011-03-14 at 15:55 +0000, Jan Beulich wrote:
> >>> On 14.03.11 at 16:19, Gianni Tedesco
<gianni.tedesco@citrix.com> wrote:
> > 
> > This permits suspend/resume to work with 32bit dom0/tools. AFAICT the
> > limit to MACH2PHYS_COMPAT_NR_ENTRIES is redundant since that refers to
a
> > limit in 32bit guest compat mappings under 64bit hypervisors, not
> > userspace where there may be gigabytes of useful virtual space
available
> > for this.
> > 
> > Suggested-by: Ian Campbell <Ian.Campbell@eu.citrix.com>
> > Signed-off-by: Gianni Tedesco <gianni.tedesco@citrix.com>
> > 
> > diff -r 8b5cbccbc654 xen/arch/x86/x86_64/compat/mm.c
> > --- a/xen/arch/x86/x86_64/compat/mm.c	Mon Mar 14 14:59:27 2011 +0000
> > +++ b/xen/arch/x86/x86_64/compat/mm.c	Mon Mar 14 15:17:59 2011 +0000
> > @@ -161,9 +161,7 @@ int compat_arch_memory_op(int op, XEN_GU
> >          if ( copy_from_guest(&xmml, arg, 1) )
> >              return -EFAULT;
> >  
> > -        limit = (unsigned long)(compat_machine_to_phys_mapping +
> > -            min_t(unsigned long, max_page,
> > -                  MACH2PHYS_COMPAT_NR_ENTRIES(current->domain)));
> > +        limit = (unsigned long)(compat_machine_to_phys_mapping +
max_page);
> 
> While doing this shouldn''t hurt (except slightly for performance
of
> the hypercall), I don''t see why it''s useful: For slots
past
> MACH2PHYS_COMPAT_NR_ENTRIES(current->domain) you
> wouldn''t read non-null page table entries anyway (up to
> RDWR_COMPAT_MPT_VIRT_END), so I don''t see why the tools
> couldn''t equally well do with what we have currently (after all
> they get told how many slots were filled).
In order to be able to migrate any guest the tools in domain 0 need to
see the entire of host M2P, not just the subset which the kernel sees
mapped into its hypervisor hole (which is what
MACH2PHYS_COMPAT_NR_ENTRIES represents).

The hypercall reads from the global compat M2P mapping, not the guest
kernel mapping of it, so it should read valid entries all the way up to
RDWR_COMPAT_MPT_VIRT_END, AFAICT.

Ian.
> 
> Jan
> 
> >          if ( limit > RDWR_COMPAT_MPT_VIRT_END )
> >              limit = RDWR_COMPAT_MPT_VIRT_END;
> >          for ( i = 0, v = RDWR_COMPAT_MPT_VIRT_START, last_mfn = 0;
> 
> 


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>>> On 14.03.11 at 17:03, Ian Campbell
<Ian.Campbell@eu.citrix.com> wrote:
> On Mon, 2011-03-14 at 15:55 +0000, Jan Beulich wrote:
>> >>> On 14.03.11 at 16:19, Gianni Tedesco
<gianni.tedesco@citrix.com> wrote:
>> > 
>> > This permits suspend/resume to work with 32bit dom0/tools. AFAICT
the
>> > limit to MACH2PHYS_COMPAT_NR_ENTRIES is redundant since that
refers to a
>> > limit in 32bit guest compat mappings under 64bit hypervisors, not
>> > userspace where there may be gigabytes of useful virtual space
available
>> > for this.
>> > 
>> > Suggested-by: Ian Campbell <Ian.Campbell@eu.citrix.com>
>> > Signed-off-by: Gianni Tedesco <gianni.tedesco@citrix.com>
>> > 
>> > diff -r 8b5cbccbc654 xen/arch/x86/x86_64/compat/mm.c
>> > --- a/xen/arch/x86/x86_64/compat/mm.c	Mon Mar 14 14:59:27 2011
+0000
>> > +++ b/xen/arch/x86/x86_64/compat/mm.c	Mon Mar 14 15:17:59 2011
+0000
>> > @@ -161,9 +161,7 @@ int compat_arch_memory_op(int op, XEN_GU
>> >          if ( copy_from_guest(&xmml, arg, 1) )
>> >              return -EFAULT;
>> >  
>> > -        limit = (unsigned long)(compat_machine_to_phys_mapping +
>> > -            min_t(unsigned long, max_page,
>> > -                 
MACH2PHYS_COMPAT_NR_ENTRIES(current->domain)));
>> > +        limit = (unsigned long)(compat_machine_to_phys_mapping + 
> max_page);
>> 
>> While doing this shouldn''t hurt (except slightly for
performance of
>> the hypercall), I don''t see why it''s useful: For
slots past
>> MACH2PHYS_COMPAT_NR_ENTRIES(current->domain) you
>> wouldn''t read non-null page table entries anyway (up to
>> RDWR_COMPAT_MPT_VIRT_END), so I don''t see why the tools
>> couldn''t equally well do with what we have currently (after
all
>> they get told how many slots were filled).
> 
> In order to be able to migrate any guest the tools in domain 0 need to
> see the entire of host M2P, not just the subset which the kernel sees
> mapped into its hypervisor hole (which is what
> MACH2PHYS_COMPAT_NR_ENTRIES represents).
> 
> The hypercall reads from the global compat M2P mapping, not the guest
> kernel mapping of it, so it should read valid entries all the way up to
> RDWR_COMPAT_MPT_VIRT_END, AFAICT.
But RDWR_COMPAT_MPT_VIRT_END still doesn''t necessarily
cover all of the memory the machine may have (after all the
range is way smaller than RDWR_MPT_VIRT_{START,END}.

If that''s the goal, then the patch as presented isn''t
suitable,
as there''s not event a compat table set up for all of the
memory. I''d say the tools then need to have access to the
native table, reading 64-bit MFNs from it (since, with MFN
compression, we can exceed 32-bits).

Jan




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On Mon, 2011-03-14 at 16:22 +0000, Jan Beulich wrote:
> >>> On 14.03.11 at 17:03, Ian Campbell
<Ian.Campbell@eu.citrix.com> wrote:
> > On Mon, 2011-03-14 at 15:55 +0000, Jan Beulich wrote:
> >> >>> On 14.03.11 at 16:19, Gianni Tedesco
<gianni.tedesco@citrix.com> wrote:
> >> > 
> >> > This permits suspend/resume to work with 32bit dom0/tools.
AFAICT the
> >> > limit to MACH2PHYS_COMPAT_NR_ENTRIES is redundant since that
refers to a
> >> > limit in 32bit guest compat mappings under 64bit hypervisors,
not
> >> > userspace where there may be gigabytes of useful virtual
space available
> >> > for this.
> >> > 
> >> > Suggested-by: Ian Campbell <Ian.Campbell@eu.citrix.com>
> >> > Signed-off-by: Gianni Tedesco
<gianni.tedesco@citrix.com>
> >> > 
> >> > diff -r 8b5cbccbc654 xen/arch/x86/x86_64/compat/mm.c
> >> > --- a/xen/arch/x86/x86_64/compat/mm.c	Mon Mar 14 14:59:27
2011 +0000
> >> > +++ b/xen/arch/x86/x86_64/compat/mm.c	Mon Mar 14 15:17:59
2011 +0000
> >> > @@ -161,9 +161,7 @@ int compat_arch_memory_op(int op, XEN_GU
> >> >          if ( copy_from_guest(&xmml, arg, 1) )
> >> >              return -EFAULT;
> >> >  
> >> > -        limit = (unsigned
long)(compat_machine_to_phys_mapping +
> >> > -            min_t(unsigned long, max_page,
> >> > -                 
MACH2PHYS_COMPAT_NR_ENTRIES(current->domain)));
> >> > +        limit = (unsigned
long)(compat_machine_to_phys_mapping +
> > max_page);
> >> 
> >> While doing this shouldn''t hurt (except slightly for
performance of
> >> the hypercall), I don''t see why it''s useful: For
slots past
> >> MACH2PHYS_COMPAT_NR_ENTRIES(current->domain) you
> >> wouldn''t read non-null page table entries anyway (up to
> >> RDWR_COMPAT_MPT_VIRT_END), so I don''t see why the tools
> >> couldn''t equally well do with what we have currently
(after all
> >> they get told how many slots were filled).
> > 
> > In order to be able to migrate any guest the tools in domain 0 need to
> > see the entire of host M2P, not just the subset which the kernel sees
> > mapped into its hypervisor hole (which is what
> > MACH2PHYS_COMPAT_NR_ENTRIES represents).
> > 
> > The hypercall reads from the global compat M2P mapping, not the guest
> > kernel mapping of it, so it should read valid entries all the way up
to
> > RDWR_COMPAT_MPT_VIRT_END, AFAICT.
> 
> But RDWR_COMPAT_MPT_VIRT_END still doesn''t necessarily
> cover all of the memory the machine may have (after all the
> range is way smaller than RDWR_MPT_VIRT_{START,END}.
It''s 1GB which is enough to cover 1TB of host memory, which AFAIK is
all
we support these days. It certainly buys us time compared with currently
failing at 160GB.
> If that''s the goal, then the patch as presented isn''t
suitable,
> as there''s not event a compat table set up for all of the
> memory.
paging_init seems to do the right thing and setup the compat M2P up to a
maximum of RDWR_COMPAT_MPT_VIRT_END.
>  I''d say the tools then need to have access to the
> native table, reading 64-bit MFNs from it (since, with MFN
> compression, we can exceed 32-bits).
That''s another option I guess.

Ian.



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On 14/03/2011 16:33, "Ian Campbell" <Ian.Campbell@eu.citrix.com>
wrote:
>> But RDWR_COMPAT_MPT_VIRT_END still doesn''t necessarily
>> cover all of the memory the machine may have (after all the
>> range is way smaller than RDWR_MPT_VIRT_{START,END}.
> 
> It''s 1GB which is enough to cover 1TB of host memory, which AFAIK
is all
> we support these days. It certainly buys us time compared with currently
> failing at 160GB.
> 
>> If that''s the goal, then the patch as presented isn''t
suitable,
>> as there''s not event a compat table set up for all of the
>> memory.
> 
> paging_init seems to do the right thing and setup the compat M2P up to a
> maximum of RDWR_COMPAT_MPT_VIRT_END.
> 
>>  I''d say the tools then need to have access to the
>> native table, reading 64-bit MFNs from it (since, with MFN
>> compression, we can exceed 32-bits).
> 
> That''s another option I guess.
It''s not really an option for 4.1.0. Can we at least agree that this is
an
improvement for now, and in time for 4.1.0?

 -- Keir



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>>> On 14.03.11 at 17:33, Ian Campbell
<Ian.Campbell@eu.citrix.com> wrote:
> On Mon, 2011-03-14 at 16:22 +0000, Jan Beulich wrote:
>> >>> On 14.03.11 at 17:03, Ian Campbell
<Ian.Campbell@eu.citrix.com> wrote:
>> > On Mon, 2011-03-14 at 15:55 +0000, Jan Beulich wrote:
>> >> >>> On 14.03.11 at 16:19, Gianni Tedesco
<gianni.tedesco@citrix.com> wrote:
>> >> > 
>> >> > This permits suspend/resume to work with 32bit
dom0/tools. AFAICT the
>> >> > limit to MACH2PHYS_COMPAT_NR_ENTRIES is redundant since
that refers to a
>> >> > limit in 32bit guest compat mappings under 64bit
hypervisors, not
>> >> > userspace where there may be gigabytes of useful virtual
space available
>> >> > for this.
>> >> > 
>> >> > Suggested-by: Ian Campbell
<Ian.Campbell@eu.citrix.com>
>> >> > Signed-off-by: Gianni Tedesco
<gianni.tedesco@citrix.com>
>> >> > 
>> >> > diff -r 8b5cbccbc654 xen/arch/x86/x86_64/compat/mm.c
>> >> > --- a/xen/arch/x86/x86_64/compat/mm.c	Mon Mar 14 14:59:27
2011 +0000
>> >> > +++ b/xen/arch/x86/x86_64/compat/mm.c	Mon Mar 14 15:17:59
2011 +0000
>> >> > @@ -161,9 +161,7 @@ int compat_arch_memory_op(int op,
XEN_GU
>> >> >          if ( copy_from_guest(&xmml, arg, 1) )
>> >> >              return -EFAULT;
>> >> >  
>> >> > -        limit = (unsigned
long)(compat_machine_to_phys_mapping +
>> >> > -            min_t(unsigned long, max_page,
>> >> > -                 
MACH2PHYS_COMPAT_NR_ENTRIES(current->domain)));
>> >> > +        limit = (unsigned
long)(compat_machine_to_phys_mapping +
>> > max_page);
>> >> 
>> >> While doing this shouldn''t hurt (except slightly for
performance of
>> >> the hypercall), I don''t see why it''s useful:
For slots past
>> >> MACH2PHYS_COMPAT_NR_ENTRIES(current->domain) you
>> >> wouldn''t read non-null page table entries anyway (up
to
>> >> RDWR_COMPAT_MPT_VIRT_END), so I don''t see why the
tools
>> >> couldn''t equally well do with what we have currently
(after all
>> >> they get told how many slots were filled).
>> > 
>> > In order to be able to migrate any guest the tools in domain 0
need to
>> > see the entire of host M2P, not just the subset which the kernel
sees
>> > mapped into its hypervisor hole (which is what
>> > MACH2PHYS_COMPAT_NR_ENTRIES represents).
>> > 
>> > The hypercall reads from the global compat M2P mapping, not the
guest
>> > kernel mapping of it, so it should read valid entries all the way
up to
>> > RDWR_COMPAT_MPT_VIRT_END, AFAICT.
>> 
>> But RDWR_COMPAT_MPT_VIRT_END still doesn''t necessarily
>> cover all of the memory the machine may have (after all the
>> range is way smaller than RDWR_MPT_VIRT_{START,END}.
> 
> It''s 1GB which is enough to cover 1TB of host memory, which AFAIK
is all
> we support these days. It certainly buys us time compared with currently
> failing at 160GB.
1Tb of *contiguous* host memory. And that''s certainly not the limit
Xen has been run on, and Xen itself is set up to handle 5Tb. Which
I''m seeing to get exceeded on experimental(?) systems...

And while I agree that failing at 1Tb is better than failing at 160Gb,
I favor fixing this once and completely over doing a little bit of
papering over the problem now just to require debugging the same
issue again later.
>> If that''s the goal, then the patch as presented isn''t
suitable,
>> as there''s not event a compat table set up for all of the
>> memory.
> 
> paging_init seems to do the right thing and setup the compat M2P up to a
> maximum of RDWR_COMPAT_MPT_VIRT_END.
With 1Gb being the theoretical limit of what a 32-bit guest can
see and access, that''s all a guest could ever sensibly ask for (a
[hypothetical] domain could ask for having a larger than the
default hole with more of the table mapped in).

Jan



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>>> On 14.03.11 at 17:54, Keir Fraser <keir.xen@gmail.com> wrote:
> On 14/03/2011 16:33, "Ian Campbell"
<Ian.Campbell@eu.citrix.com> wrote:
> 
>>> But RDWR_COMPAT_MPT_VIRT_END still doesn''t necessarily
>>> cover all of the memory the machine may have (after all the
>>> range is way smaller than RDWR_MPT_VIRT_{START,END}.
>> 
>> It''s 1GB which is enough to cover 1TB of host memory, which
AFAIK is all
>> we support these days. It certainly buys us time compared with
currently
>> failing at 160GB.
>> 
>>> If that''s the goal, then the patch as presented
isn''t suitable,
>>> as there''s not event a compat table set up for all of the
>>> memory.
>> 
>> paging_init seems to do the right thing and setup the compat M2P up to
a
>> maximum of RDWR_COMPAT_MPT_VIRT_END.
>> 
>>>  I''d say the tools then need to have access to the
>>> native table, reading 64-bit MFNs from it (since, with MFN
>>> compression, we can exceed 32-bits).
>> 
>> That''s another option I guess.
> 
> It''s not really an option for 4.1.0. Can we at least agree that
this is an
> improvement for now, and in time for 4.1.0?
Yes, as long as the tools can handle the extended output.

Jan


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On Mon, 2011-03-14 at 17:00 +0000, Jan Beulich wrote:
> >>> On 14.03.11 at 17:54, Keir Fraser <keir.xen@gmail.com>
wrote:
> > On 14/03/2011 16:33, "Ian Campbell"
<Ian.Campbell@eu.citrix.com> wrote:
> > 
> >>> But RDWR_COMPAT_MPT_VIRT_END still doesn''t
necessarily
> >>> cover all of the memory the machine may have (after all the
> >>> range is way smaller than RDWR_MPT_VIRT_{START,END}.
> >> 
> >> It''s 1GB which is enough to cover 1TB of host memory,
which AFAIK is all
> >> we support these days. It certainly buys us time compared with
currently
> >> failing at 160GB.
> >> 
> >>> If that''s the goal, then the patch as presented
isn''t suitable,
> >>> as there''s not event a compat table set up for all of
the
> >>> memory.
> >> 
> >> paging_init seems to do the right thing and setup the compat M2P
up to a
> >> maximum of RDWR_COMPAT_MPT_VIRT_END.
> >> 
> >>>  I''d say the tools then need to have access to the
> >>> native table, reading 64-bit MFNs from it (since, with MFN
> >>> compression, we can exceed 32-bits).
> >> 
> >> That''s another option I guess.
> > 
> > It''s not really an option for 4.1.0. Can we at least agree
that this is an
> > improvement for now, and in time for 4.1.0?
> 
> Yes, as long as the tools can handle the extended output.
It certainly seems to work for the domain suspend case, I''m not sure
where else it might fail but it-seems-to-me(tm) that tools should be
able to handle the output that they asked for.

Gianni


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On Mon, 2011-03-14 at 16:58 +0000, Jan Beulich wrote:
> 
> > It''s 1GB which is enough to cover 1TB of host memory, which
AFAIK is
> all
> > we support these days. It certainly buys us time compared with
> currently
> > failing at 160GB.
> 
> 1Tb of *contiguous* host memory. And that''s certainly not the
limit
> Xen has been run on, and Xen itself is set up to handle 5Tb. Which
> I''m seeing to get exceeded on experimental(?) systems...
> 
> And while I agree that failing at 1Tb is better than failing at 160Gb,
> I favor fixing this once and completely over doing a little bit of
> papering over the problem now just to require debugging the same
> issue again later.
Maybe a comment or a printk() would be advisable here then in the mean
time?

Gianni


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On Mon, 2011-03-14 at 16:58 +0000, Jan Beulich wrote:
> >>> On 14.03.11 at 17:33, Ian Campbell
<Ian.Campbell@eu.citrix.com> wrote:
> > On Mon, 2011-03-14 at 16:22 +0000, Jan Beulich wrote:
> >> >>> On 14.03.11 at 17:03, Ian Campbell
<Ian.Campbell@eu.citrix.com> wrote:
> >> > On Mon, 2011-03-14 at 15:55 +0000, Jan Beulich wrote:
> >> >> >>> On 14.03.11 at 16:19, Gianni Tedesco
<gianni.tedesco@citrix.com> wrote:
> >> >> > 
> >> >> > This permits suspend/resume to work with 32bit
dom0/tools. AFAICT the
> >> >> > limit to MACH2PHYS_COMPAT_NR_ENTRIES is redundant
since that refers to a
> >> >> > limit in 32bit guest compat mappings under 64bit
hypervisors, not
> >> >> > userspace where there may be gigabytes of useful
virtual space available
> >> >> > for this.
> >> >> > 
> >> >> > Suggested-by: Ian Campbell
<Ian.Campbell@eu.citrix.com>
> >> >> > Signed-off-by: Gianni Tedesco
<gianni.tedesco@citrix.com>
> >> >> > 
> >> >> > diff -r 8b5cbccbc654 xen/arch/x86/x86_64/compat/mm.c
> >> >> > --- a/xen/arch/x86/x86_64/compat/mm.c	Mon Mar 14
14:59:27 2011 +0000
> >> >> > +++ b/xen/arch/x86/x86_64/compat/mm.c	Mon Mar 14
15:17:59 2011 +0000
> >> >> > @@ -161,9 +161,7 @@ int compat_arch_memory_op(int
op, XEN_GU
> >> >> >          if ( copy_from_guest(&xmml, arg, 1) )
> >> >> >              return -EFAULT;
> >> >> >  
> >> >> > -        limit = (unsigned
long)(compat_machine_to_phys_mapping +
> >> >> > -            min_t(unsigned long, max_page,
> >> >> > -                 
MACH2PHYS_COMPAT_NR_ENTRIES(current->domain)));
> >> >> > +        limit = (unsigned
long)(compat_machine_to_phys_mapping +
> >> > max_page);
> >> >> 
> >> >> While doing this shouldn''t hurt (except slightly
for performance of
> >> >> the hypercall), I don''t see why it''s
useful: For slots past
> >> >> MACH2PHYS_COMPAT_NR_ENTRIES(current->domain) you
> >> >> wouldn''t read non-null page table entries anyway
(up to
> >> >> RDWR_COMPAT_MPT_VIRT_END), so I don''t see why
the tools
> >> >> couldn''t equally well do with what we have
currently (after all
> >> >> they get told how many slots were filled).
> >> > 
> >> > In order to be able to migrate any guest the tools in domain
0 need to
> >> > see the entire of host M2P, not just the subset which the
kernel sees
> >> > mapped into its hypervisor hole (which is what
> >> > MACH2PHYS_COMPAT_NR_ENTRIES represents).
> >> > 
> >> > The hypercall reads from the global compat M2P mapping, not
the guest
> >> > kernel mapping of it, so it should read valid entries all the
way up to
> >> > RDWR_COMPAT_MPT_VIRT_END, AFAICT.
> >> 
> >> But RDWR_COMPAT_MPT_VIRT_END still doesn''t necessarily
> >> cover all of the memory the machine may have (after all the
> >> range is way smaller than RDWR_MPT_VIRT_{START,END}.
> > 
> > It''s 1GB which is enough to cover 1TB of host memory, which
AFAIK is all
> > we support these days. It certainly buys us time compared with
currently
> > failing at 160GB.
> 
> 1Tb of *contiguous* host memory. And that''s certainly not the
limit
> Xen has been run on, and Xen itself is set up to handle 5Tb. Which
> I''m seeing to get exceeded on experimental(?) systems...
Cool, I stand corrected.
> And while I agree that failing at 1Tb is better than failing at 160Gb,
> I favor fixing this once and completely over doing a little bit of
> papering over the problem now just to require debugging the same
> issue again later.
Unfortunately it''s a bit late to be doing that for 4.1.0 :-(
> >> If that''s the goal, then the patch as presented
isn''t suitable,
> >> as there''s not event a compat table set up for all of the
> >> memory.
> > 
> > paging_init seems to do the right thing and setup the compat M2P up to
a
> > maximum of RDWR_COMPAT_MPT_VIRT_END.
> 
> With 1Gb being the theoretical limit of what a 32-bit guest can
> see and access, that''s all a guest could ever sensibly ask for (a
> [hypothetical] domain could ask for having a larger than the
> default hole with more of the table mapped in).
The size of a domain''s hypervisor hole and how much of the m2p it can
map via XENMEM_machphys_mfn_list have no relationship though -- that''s
the point of this change.

Ian.


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>>> On 15.03.11 at 16:00, Ian Campbell
<Ian.Campbell@eu.citrix.com> wrote:
> On Mon, 2011-03-14 at 16:58 +0000, Jan Beulich wrote:
>> >>> On 14.03.11 at 17:33, Ian Campbell
<Ian.Campbell@eu.citrix.com> wrote:
>> > paging_init seems to do the right thing and setup the compat M2P
up to a
>> > maximum of RDWR_COMPAT_MPT_VIRT_END.
>> 
>> With 1Gb being the theoretical limit of what a 32-bit guest can
>> see and access, that''s all a guest could ever sensibly ask for
(a
>> [hypothetical] domain could ask for having a larger than the
>> default hole with more of the table mapped in).
> 
> The size of a domain''s hypervisor hole and how much of the m2p it
can
> map via XENMEM_machphys_mfn_list have no relationship though --
that''s
> the point of this change.
I understand that (now) - I was just trying to explain why things
were coded to setup the full range, but expose only what would
be visible through the hole.

Jan


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On Tue, 2011-03-15 at 15:09 +0000, Jan Beulich wrote:
> >>> On 15.03.11 at 16:00, Ian Campbell
<Ian.Campbell@eu.citrix.com> wrote:
> > On Mon, 2011-03-14 at 16:58 +0000, Jan Beulich wrote:
> >> >>> On 14.03.11 at 17:33, Ian Campbell
<Ian.Campbell@eu.citrix.com> wrote:
> >> > paging_init seems to do the right thing and setup the compat
M2P up to a
> >> > maximum of RDWR_COMPAT_MPT_VIRT_END.
> >> 
> >> With 1Gb being the theoretical limit of what a 32-bit guest can
> >> see and access, that''s all a guest could ever sensibly
ask for (a
> >> [hypothetical] domain could ask for having a larger than the
> >> default hole with more of the table mapped in).
> > 
> > The size of a domain''s hypervisor hole and how much of the
m2p it can
> > map via XENMEM_machphys_mfn_list have no relationship though --
that''s
> > the point of this change.
> 
> I understand that (now) - I was just trying to explain why things
> were coded to setup the full range, but expose only what would
> be visible through the hole.
Ah, right, makes sense.

Thanks,
Ian,



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