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RE: [Xen-devel] Xenoprof in an HVM domain

To: "Ray Bryant" <raybry@xxxxxxxxxxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: RE: [Xen-devel] Xenoprof in an HVM domain
From: "Santos, Jose Renato G" <joserenato.santos@xxxxxx>
Date: Thu, 25 May 2006 09:44:15 -0700
Cc: "Eranian, Stephane" <stephane.eranian@xxxxxx>, Steve Dobbelstein <steved@xxxxxxxxxx>
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Thread-topic: [Xen-devel] Xenoprof in an HVM domain

>> -----Original Message-----
>> From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx 
>> [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of 
>> Ray Bryant
>> Sent: Thursday, May 25, 2006 7:21 AM
>> To: xen-devel@xxxxxxxxxxxxxxxxxxx
>> Cc: Steve Dobbelstein; Eranian, Stephane
>> Subject: Re: [Xen-devel] Xenoprof in an HVM domain
   ... <stuff deleted> ....
>> This assumes, of course, that one can figure out 
>> how to virtualize the performance counters, and then the 
>> hypervisor would have to sort out conflicts, etc, between 
>> domains that wanted to use the performance counters (perhaps 
>> these would be a resource the hypervisor could dynamically 
>> allocate to a domain, by, for 
>> example, some kind of "xm" command).    

I don't think this is how performance counters should be virtualized.
Virtualizing performance counter should save/restore the values of
active perf counters on every VCPU/domain context switch. There should
be no need for such a "xm" command.

Performance counter virtualization is not currently supported in Xen,
although it would be nice to have it. With counter virtualization, guest
domains would be able to profile themselves with un-modified oprofile.
This would be usefull to enable users to profile their applications on
Xen guests in the same way they are used to do on vanila linux.

The current model supported by Xenoprof is system-wide profiling, where
counters are used to profile the collection of domains and Xen together.
This is usefull for Xen developers to optimize Xen and para-virtualized
kernels running on Xen.

Ideally we would like to have support for both system-wide profiling
(for Xen developers) and independent guest profiling with perf counter
virtualization (for Xen users). Adding perf counter virtualization is in
our to do list. If anybody is interested in working on this please let
me know.
We would appreciate any help we could get.


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