Xen devel - Oct 2005 - [PATCH] Bug #327 xentop/libxenstat vcpu fix

xentop will error when getting vcpu information if there are disabled
vcpus for a domain at the end of the vcpu list. xentop merely goes
through the num_vcpus list and calls DOM0_GETVCPUCONTEXT, it doesn''t
check if the requested vcpu is disabled or enabled. DOM0_GETVCPUCONTEXT
returns the next enabled vcpu in the list or an error if it doesn''t
find
one. xentop will then error.

This is just a quick fix to check the vcpu_to_cpu[] map before
requesting vcpu info. 

Signed-off-by: Daniel Stekloff <dsteklof@us.ibm.com>




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On 14 Oct 2005, at 23:39, Daniel Stekloff wrote:
> xentop will error when getting vcpu information if there are disabled
> vcpus for a domain at the end of the vcpu list. xentop merely goes
> through the num_vcpus list and calls DOM0_GETVCPUCONTEXT, it
doesn''t
> check if the requested vcpu is disabled or enabled. DOM0_GETVCPUCONTEXT
> returns the next enabled vcpu in the list or an error if it
doesn''t
> find
> one. xentop will then error.
>
> This is just a quick fix to check the vcpu_to_cpu[] map before
> requesting vcpu info.
GETVCPUCONTEXT no longer returns the next valid vcpu. It just fails if 
the specified vcpu is not currently online. Also, GETDOMAININFO no 
longer returns vcpu_to_cpu or cpumap arrays. I moved those into a new 
GETVCPUINFO call.

I think you can use a simpler algorithm for finding all online VCPUs 
anyway. The tools will always try to ensure that VCPUs 0 to 
info.n_vcpu-1 are online, so you could just iterate over that. If you 
are worried about discontiguous online maps, just iterate over 
0-(MAX_VIRT_CPUS-1) and silently drop the GETVCPUINFO failures 
(errno==ESRCH).

  -- Keir


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On Saturday 15 October 2005 00:44, Keir Fraser wrote:
> On 14 Oct 2005, at 23:39, Daniel Stekloff wrote:
> > xentop will error when getting vcpu information if there are disabled
> > vcpus for a domain at the end of the vcpu list. xentop merely goes
> > through the num_vcpus list and calls DOM0_GETVCPUCONTEXT, it
doesn''t
> > check if the requested vcpu is disabled or enabled.
DOM0_GETVCPUCONTEXT
> > returns the next enabled vcpu in the list or an error if it
doesn''t
> > find
> > one. xentop will then error.
> >
> > This is just a quick fix to check the vcpu_to_cpu[] map before
> > requesting vcpu info.
>
> GETVCPUCONTEXT no longer returns the next valid vcpu. It just fails if
> the specified vcpu is not currently online. Also, GETDOMAININFO no
> longer returns vcpu_to_cpu or cpumap arrays. I moved those into a new
> GETVCPUINFO call.
>
> I think you can use a simpler algorithm for finding all online VCPUs
> anyway. The tools will always try to ensure that VCPUs 0 to
> info.n_vcpu-1 are online, so you could just iterate over that. If you
> are worried about discontiguous online maps, just iterate over
> 0-(MAX_VIRT_CPUS-1) and silently drop the GETVCPUINFO failures
> (errno==ESRCH).

Ok.. so xm top/xentop is no longer broken - provided you don''t care
about out
of order vcpus. 

xm vcpu-list is broken.

xm vcpu-enable will only enable vcpus in order.  I can''t, for instance,
enable
vcpu 3 if vcpu 2 isn''t enabled. xm vcpu-enable silently fails. 

So stupid question: 

Do we no longer have or care about dicontiguous online maps? Or, do the tools 
just need to catch up with your changes? 

I''m curious because xentop just takes the number of vcpus from
domaininfo and
calls GETVCPUCONTEXT starting with 0 and going until num-1. I just tried this 
on a 4 vcpu dom. I disabled vcpus 1 and 2, leaving 0 and 3 active. xentop 
worked fine but reported those vcpus as 0 and 1. GETVCPUCONTEXT returned the 
as 0 and 1. Is this the way you want it to work? 

If so, I''ll close the bug. 

Thanks,

Dan

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On 16 Oct 2005, at 01:11, Daniel Stekloff wrote:
> Ok.. so xm top/xentop is no longer broken - provided you don''t
care
> about out
> of order vcpus.
This doesn''t make sense..... see below.
> xm vcpu-list is broken.
Yes, Ewan is working on a fix for this.
> xm vcpu-enable will only enable vcpus in order.  I can''t, for 
> instance, enable
> vcpu 3 if vcpu 2 isn''t enabled. xm vcpu-enable silently fails.
This is currently supposed to work. It looks as though the ''xm 
vcpu-enable/disable'' commands are still not 100% robust. :-( Longer 
term, allowing an admin to take down specific vcpus probably isn''t all 
that useful, so we may simplify the xm interface.
> Do we no longer have or care about dicontiguous online maps? Or, do 
> the tools
> just need to catch up with your changes?
Although we don''t care about exposing them via vcpu-enable/disable, I 
think we ought to handle them. A user logged into domU can still take 
down non-contiguous VCPUs, even if that functionality isn''t exported 
via the xm interface.
> I''m curious because xentop just takes the number of vcpus from 
> domaininfo and
> calls GETVCPUCONTEXT starting with 0 and going until num-1. I just 
> tried this
> on a 4 vcpu dom. I disabled vcpus 1 and 2, leaving 0 and 3 active. 
> xentop
> worked fine but reported those vcpus as 0 and 1. GETVCPUCONTEXT 
> returned the
> as 0 and 1. Is this the way you want it to work?
That is definitely not how GETVCPUCONTEXT works. It won''t renumber 
vcpus under your feet  -- if you ask for vcpu3 then it is vcpu3''s 
information that you will get.

One thing to bear in mind: GETVCPUCONTEXT will return info even about 
vcpus that are not currently online. To check for online status you 
need to use GETVCPUINFO and check the online field that is returned.

Perhaps you iterated from 0 to info.n_vcpu-1? The count from 
GETDOMAININFO only counts online vcpus, but doesn;t tell you which 
vcpus are online. We could also add a count ''max_vcpu'' or
easier: just
iterate 0 to MAX_VIRT_CPU-1.

  -- Keir


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On Sunday 16 October 2005 01:38, Keir Fraser wrote:
> On 16 Oct 2005, at 01:11, Daniel Stekloff wrote:
> > Ok.. so xm top/xentop is no longer broken - provided you
don''t care
> > about out
> > of order vcpus.
>
> This doesn''t make sense..... see below.
>
> > xm vcpu-list is broken.
>
> Yes, Ewan is working on a fix for this.
>
> > xm vcpu-enable will only enable vcpus in order.  I can''t, for
> > instance, enable
> > vcpu 3 if vcpu 2 isn''t enabled. xm vcpu-enable silently
fails.
>
> This is currently supposed to work. It looks as though the ''xm
> vcpu-enable/disable'' commands are still not 100% robust. :-(
Longer
> term, allowing an admin to take down specific vcpus probably isn''t
all
> that useful, so we may simplify the xm interface.
>
> > Do we no longer have or care about dicontiguous online maps? Or, do
> > the tools
> > just need to catch up with your changes?
>
> Although we don''t care about exposing them via
vcpu-enable/disable, I
> think we ought to handle them. A user logged into domU can still take
> down non-contiguous VCPUs, even if that functionality isn''t
exported
> via the xm interface.
>
> > I''m curious because xentop just takes the number of vcpus
from
> > domaininfo and
> > calls GETVCPUCONTEXT starting with 0 and going until num-1. I just
> > tried this
> > on a 4 vcpu dom. I disabled vcpus 1 and 2, leaving 0 and 3 active.
> > xentop
> > worked fine but reported those vcpus as 0 and 1. GETVCPUCONTEXT
> > returned the
> > as 0 and 1. Is this the way you want it to work?
>
> That is definitely not how GETVCPUCONTEXT works. It won''t renumber
> vcpus under your feet  -- if you ask for vcpu3 then it is vcpu3''s
> information that you will get.
>
> One thing to bear in mind: GETVCPUCONTEXT will return info even about
> vcpus that are not currently online. To check for online status you
> need to use GETVCPUINFO and check the online field that is returned.
>
> Perhaps you iterated from 0 to info.n_vcpu-1? The count from
> GETDOMAININFO only counts online vcpus, but doesn;t tell you which
> vcpus are online. We could also add a count ''max_vcpu'' or
easier: just
> iterate 0 to MAX_VIRT_CPU-1.
>

Sorry, I should have looked at the changes more closely, I didn''t
realize:

1) getdomaininfo returns number of active cpus

2) getvcpucontext will now return info about vpcus that aren''t active.

I can certainly change xentop to iterate over a domain''s vcpus bounded
by
MAX_VIRT_CPU and number of active vcpus, calling getvcpuinfo along the way. 
It had been using the info.n_vcpu as a bound. 

Just as a suggestion, it might be nicer for tools to know max_vcpus rather 
than having to iterate over the entire possible list when they want to know 
how many (enabled and disabled) vcpus a domain has. Say, if someone wanted to 
know how may vcpus have been configured rather than checking the config file. 
It also may be more efficient - less GETVCPUINFO calls. 

Thanks for your help.

Dan

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