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

On 08/03/13 14:50, Boris Ostrovsky wrote:
----- JBeulich@xxxxxxxx wrote:

On 04.03.13 at 13:42, George Dunlap <George.Dunlap@xxxxxxxxxxxxx>
On Fri, Mar 1, 2013 at 8:49 PM,  <suravee.suthikulpanit@xxxxxxx>
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
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.


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