Re: [PATCH 2/3] perf, x86: Use GO/HO bits in perf-ctr

[Joerg Roedel <joro@8bytes.org>]

On Wed, May 11, 2011 at 03:58:34PM +0300, Avi Kivity wrote:
> On 05/10/2011 06:04 PM, Roedel, Joerg wrote:
>> On Tue, May 10, 2011 at 10:48:20AM -0400, Peter Zijlstra wrote:
>> >  On Tue, 2011-05-10 at 16:35 +0200, Joerg Roedel wrote:
>> >  >  The AMD perf-counters support counting in guest or host-mode
>> >  >  only. Make use of that feature when user-space specified
>> >  >  guest/host-mode only counting.
>> >
>> >  Subject mentions x86, does Intel have anything similar so you can make
>> >  it work for them too?
>>
>> Intel does not support guest or host-only counting in the hardware (at
>> least according to my documentation). If wanted it could be approximated by
>> enabling/disabling the counters in the guest-entry path.
>
> vmx has support for atomically swapping MSRs during guest entry and exit  
> (you can load guest MSRs on entry, save guest MSRs on exit, and load  
> host MSRs on exit, but you can't save host MSRs on entry, so host-only  
> counters cannot be 100% accurate).  We'd need some kvm/perf hooks to  
> program these MSR swaps, and to manually save the counters that cannot  
> be done automatically.

Well, wenn host counters are not saved on vmentry then we just need to
save them manually (which is a one-time thing and does not need to
happen at every vmentry).
Thanks for that information, when I am back in office on monday I'll try
to grab a machine and hopefully get this running.

> btw, your patchset can be further improved by integrating  
> exclude_guest/exclude_host into the constraints.  For example if we have  
> three general purpose counters, two generic perf_events in user, one  
> exclude_guest perf_event, and one exclude_host perf_event, we can  
> schedule them all at all times, swapping the exclude_guest and  
> exclude_host events during guest entry/exit.

Well, it will need some additional code in the amd vmrun path too. I'll
take a look at it, but probably leave it as a future optimization.
Thanks for the hint.

	Joerg