thr3ads.net - Xen devel - [Xen-devel] Re: what''s the purpose of linear page table (PML4 entry 258) [Mar 2008]

If this information is useful, please help other people find it:
Share via:

weiming

2008-Mar-31 18:15 UTC

[Xen-devel] Re: what''s the purpose of linear page table (PML4 entry 258)

I find that it give the same pte value as I manually walking the 4 level
page table directory.
So I guest it facilitates the locating of a pte from va. It removes the
4-level looking up process: pgd->pud->pmd->pt->pte
It saves time. Am I right?

But I still don''t know how it works.

On Mon, Mar 31, 2008 at 2:41 AM, weiming <zephyr.zhao@gmail.com> wrote:
> Hi,
>
> When I read the code of do_update_va_mapping, I find that it update the
> pte entry in the line page table by looking up the line page table via
> guest_map_l1e() :
>
> 2998
<http://lxr.xensource.com/lxr/source/xen/arch/x86/mm.c?a=x86_64#L2998> int
do_update_va_mapping
<http://lxr.xensource.com/lxr/ident?a=x86_64;i=do_update_va_mapping>(unsigned
long va, u64 <http://lxr.xensource.com/lxr/ident?a=x86_64;i=u64> val64,
> 2999
<http://lxr.xensource.com/lxr/source/xen/arch/x86/mm.c?a=x86_64#L2999>    
unsigned long flags <http://lxr.xensource.com/lxr/ident?a=x86_64;i=flags>)
> 3000
<http://lxr.xensource.com/lxr/source/xen/arch/x86/mm.c?a=x86_64#L3000> {
> 3001
<http://lxr.xensource.com/lxr/source/xen/arch/x86/mm.c?a=x86_64#L3001>    
l1_pgentry_t <http://lxr.xensource.com/lxr/ident?a=x86_64;i=l1_pgentry_t> 
val <http://lxr.xensource.com/lxr/ident?a=x86_64;i=val> = l1e_from_intpte
<http://lxr.xensource.com/lxr/ident?a=x86_64;i=l1e_from_intpte>(val64);
> 3002
<http://lxr.xensource.com/lxr/source/xen/arch/x86/mm.c?a=x86_64#L3002>    
struct vcpu <http://lxr.xensource.com/lxr/ident?a=x86_64;i=vcpu>   *v
<http://lxr.xensource.com/lxr/ident?a=x86_64;i=v>   = current
<http://lxr.xensource.com/lxr/ident?a=x86_64;i=current>;
> 3003
<http://lxr.xensource.com/lxr/source/xen/arch/x86/mm.c?a=x86_64#L3003>    
struct domain <http://lxr.xensource.com/lxr/ident?a=x86_64;i=domain> *d  
= v <http://lxr.xensource.com/lxr/ident?a=x86_64;i=v>->domain
<http://lxr.xensource.com/lxr/ident?a=x86_64;i=domain>;
> 3004
<http://lxr.xensource.com/lxr/source/xen/arch/x86/mm.c?a=x86_64#L3004>    
l1_pgentry_t <http://lxr.xensource.com/lxr/ident?a=x86_64;i=l1_pgentry_t> 
*pl1e;
> 3005
<http://lxr.xensource.com/lxr/source/xen/arch/x86/mm.c?a=x86_64#L3005>    
unsigned long  vmask, bmap_ptr, gl1mfn;
> 3006
<http://lxr.xensource.com/lxr/source/xen/arch/x86/mm.c?a=x86_64#L3006>    
cpumask_t <http://lxr.xensource.com/lxr/ident?a=x86_64;i=cpumask_t>     
pmask;
> 3007
<http://lxr.xensource.com/lxr/source/xen/arch/x86/mm.c?a=x86_64#L3007>    
int            rc <http://lxr.xensource.com/lxr/ident?a=x86_64;i=rc>  = 0;
> 3008
<http://lxr.xensource.com/lxr/source/xen/arch/x86/mm.c?a=x86_64#L3008>
> 3009
<http://lxr.xensource.com/lxr/source/xen/arch/x86/mm.c?a=x86_64#L3009>    
perfc_incr
<http://lxr.xensource.com/lxr/ident?a=x86_64;i=perfc_incr>(calls_to_update_va);
> 3010
<http://lxr.xensource.com/lxr/source/xen/arch/x86/mm.c?a=x86_64#L3010>
> 3011
<http://lxr.xensource.com/lxr/source/xen/arch/x86/mm.c?a=x86_64#L3011>    
if ( unlikely
<http://lxr.xensource.com/lxr/ident?a=x86_64;i=unlikely>(!__addr_ok
<http://lxr.xensource.com/lxr/ident?a=x86_64;i=__addr_ok>(va) &&
!paging_mode_external
<http://lxr.xensource.com/lxr/ident?a=x86_64;i=paging_mode_external>(d)) )
> 3012
<http://lxr.xensource.com/lxr/source/xen/arch/x86/mm.c?a=x86_64#L3012>    
return -EINVAL <http://lxr.xensource.com/lxr/ident?a=x86_64;i=EINVAL>;
> 3013
<http://lxr.xensource.com/lxr/source/xen/arch/x86/mm.c?a=x86_64#L3013>
> 3014
<http://lxr.xensource.com/lxr/source/xen/arch/x86/mm.c?a=x86_64#L3014>    
rc <http://lxr.xensource.com/lxr/ident?a=x86_64;i=rc> =
xsm_update_va_mapping
<http://lxr.xensource.com/lxr/ident?a=x86_64;i=xsm_update_va_mapping>(current
<http://lxr.xensource.com/lxr/ident?a=x86_64;i=current>->domain
<http://lxr.xensource.com/lxr/ident?a=x86_64;i=domain>, val
<http://lxr.xensource.com/lxr/ident?a=x86_64;i=val>);
> 3015
<http://lxr.xensource.com/lxr/source/xen/arch/x86/mm.c?a=x86_64#L3015>    
if ( rc <http://lxr.xensource.com/lxr/ident?a=x86_64;i=rc> )
> 3016
<http://lxr.xensource.com/lxr/source/xen/arch/x86/mm.c?a=x86_64#L3016>    
return rc <http://lxr.xensource.com/lxr/ident?a=x86_64;i=rc>;
> 3017
<http://lxr.xensource.com/lxr/source/xen/arch/x86/mm.c?a=x86_64#L3017>
> 3018
<http://lxr.xensource.com/lxr/source/xen/arch/x86/mm.c?a=x86_64#L3018>    
LOCK_BIGLOCK
<http://lxr.xensource.com/lxr/ident?a=x86_64;i=LOCK_BIGLOCK>(d);
> 3019
<http://lxr.xensource.com/lxr/source/xen/arch/x86/mm.c?a=x86_64#L3019>
> 3020
<http://lxr.xensource.com/lxr/source/xen/arch/x86/mm.c?a=x86_64#L3020>    
pl1e = guest_map_l1e
<http://lxr.xensource.com/lxr/ident?a=x86_64;i=guest_map_l1e>(v
<http://lxr.xensource.com/lxr/ident?a=x86_64;i=v>, va, &gl1mfn);
>
> ...
> ...
>
>
>
> what''s the purpose of this table?
>
> Thanks
>

_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xensource.com
http://lists.xensource.com/xen-devel

Tim Deegan

2008-Apr-01 08:47 UTC

head link

Re: [Xen-devel] Re: what''s the purpose of linear page table (PML4 entry 258)

At 14:15 -0400 on 31 Mar (1206972951), weiming wrote:> I find that it give the same pte value as I manually walking the 4 level
> page table directory.
> So I guest it facilitates the locating of a pte from va. It removes the
> 4-level looking up process: pgd->pud->pmd->pt->pte
> It saves time. Am I right?
IIRC, it just does that walk internally and hands back the answer.  I
don''t recall off the top of my head why Michael introduced it except
that it was necessary for shadowing some kinds of PV guests.  It may
have been to do with the way the shadow-pagetable spinlocks used to
work.

Tim.

-- 
Tim Deegan <Tim.Deegan@citrix.com>
Principal Software Engineer, Citrix Systems (R&D) Ltd.
[Company #02300071, SL9 0DZ, UK.]

_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xensource.com
http://lists.xensource.com/xen-devel

Tim Deegan

2008-Apr-01 08:52 UTC

head link

Re: [Xen-devel] Re: what''s the purpose of linear page table (PML4 entry 258)

At 08:47 +0000 on 01 Apr (1207039678), Tim Deegan wrote:> At 14:15 -0400 on 31 Mar (1206972951), weiming wrote:
> > I find that it give the same pte value as I manually walking the 4
level
> > page table directory.
> > So I guest it facilitates the locating of a pte from va. It removes
the
> > 4-level looking up process: pgd->pud->pmd->pt->pte
> > It saves time. Am I right?
> 
> IIRC, it just does that walk internally and hands back the answer. 
Excuse me, not enough coffee.  Yes, it uses the linear mapping of the
pagetable to save time.

Tim.

-- 
Tim Deegan <Tim.Deegan@citrix.com>
Principal Software Engineer, Citrix Systems (R&D) Ltd.
[Company #02300071, SL9 0DZ, UK.]

_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xensource.com
http://lists.xensource.com/xen-devel

Xen devel - Mar 2008 - Re: what''s the purpose of linear page table (PML4 entry 258)

[Xen-devel] Re: what''s the purpose of linear page table (PML4 entry 258)

Re: [Xen-devel] Re: what''s the purpose of linear page table (PML4 entry 258)

Re: [Xen-devel] Re: what''s the purpose of linear page table (PML4 entry 258)