Re: 8bf00a529967dafbbb210b377c38a15834d1e979 - performance regression?

Michael S. Tsirkin
2013-11-04T20:08:40Z

On Thu, Oct 31, 2013 at 09:48:08AM +0200, Gleb Natapov wrote:
> On Thu, Oct 31, 2013 at 02:21:46AM +0200, Michael S. Tsirkin wrote:
> > commit 8bf00a529967dafbbb210b377c38a15834d1e979:
> > " KVM: VMX: add support for switching of PERF_GLOBAL_CTRL " was
> > as far as I can tell supposed to bring about performance improvement
> > on hardware that supports it?
> No, it (and commits after it) supposed to fix a bug which it did.
>
> > Instead it seems to make the typical case (not running guest
> > under perf) a bit slower than it used to be.
> > the cost of VMexit goes up by about 50 cycles
> > on sandy bridge where the optimization in question
> > actually is activated.
> >
> You seams to be confused. 8bf00a529967dafbbb210 adds support for special
> PERF_GLOBAL_CTRL switching, but does not add code to switch anything,
> so the commit itself is a nop.

It does add code to add_atomic_switch_msr.

> Next commit d7cd97964ba6d70c5
> uses add_atomic_switch_msr()/clear_atomic_switch_msr()
> to switch PERF_GLOBAL_CTRL, but it does not depend on
> VM_(ENTRY|EXIT)_LOAD_IA32_PERF_GLOBAL_CTRL support which previous
> patch added, if the support is not there the switching will use
> another mechanism which is even slower. So MSR is switched no matter
> if PERF_GLOBAL_CTRL is enabled or not. If you saying that using
> VM_(ENTRY|EXIT)_LOAD_IA32_PERF_GLOBAL_CTRL is slower than using generic
> vmentry MSR switching then I pretty much doubt it since the only purpose
> of special VM_(ENTRY|EXIT)_LOAD_IA32_PERF_GLOBAL_CTRL is to be faster
> then general mechanism.
>
> --
> Gleb.
> --
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