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

[George Dunlap <george.dunlap@xxxxxxxxxxxxx>]

On 08/03/13 14:50, Boris Ostrovsky wrote:
> ----- JBeulich@xxxxxxxx wrote:
>
> > > > > On 04.03.13 at 13:42, George Dunlap <George.Dunlap@xxxxxxxxxxxxx>
> > wrote:
> > > On Fri, Mar 1, 2013 at 8:49 PM,  <suravee.suthikulpanit@xxxxxxx>
> > wrote:
> > > > From: Suravee Suthikulpanit <suravee.suthikulpanit@xxxxxxx>
> > > >
> > > > 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


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