Re: [PATCH] Always save/restore performance counters when HVM guest switching VCPU

[George Dunlap <george.dunlap@eu.citrix.com>]

On 08/03/13 14:50, Boris Ostrovsky wrote:
> ----- JBeulich@suse.com wrote:
>
>>>>> On 04.03.13 at 13:42, George Dunlap <George.Dunlap@eu.citrix.com>
>> wrote:
>>> On Fri, Mar 1, 2013 at 8:49 PM,  <suravee.suthikulpanit@amd.com>
>> wrote:
>>>> From: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
>>>>
>>>> Currently, the performance counter registers are saved/restores
>>>> when the HVM guest switchs VCPUs only if they are running.
>>>> However, PERF has one check where it writes the MSR and read back
>>>> the value to check if the MSR is working.  This has shown to fails
>>>> the check if the VCPU is moved in between rdmsr and wrmsr and
>>>> resulting in the values are different.
>>> Many moons ago (circa 2005) when I used performance counters, I
>> found
>>> that adding them to the save/restore path added a non-neligible
>>> overhead -- something like 5% slow-down.  Do you have any reason to
>>> believe this is no longer the case?  Have you done any benchmarks
>>> before and after?
> I was doing some VPMU tracing a couple of weeks ago and by looking at
> trace timestamps I think I saw about 4000 cycles on VPMU save and
> ~9000 cycles on restore. Don't remember what it was percentage-wise of
> a whole context switch.
>
> This was on Intel.

That's a really hefty expense to make all users pay on every context 
switch, on behalf of a random check in a piece of software that only a 
handful of people are going to be actually using.

I'm having a hard time telling what PERF is being talked about here -- 
couldn't this check be fixed on their side, by perhaps checking the 
CPUID leaf for the existence of Xen?

If not I think a "lazy vpmu activation" is going to be the only option.

  -George