Re: 8bf00a529967dafbbb210b377c38a15834d1e979 - performance regression?

Gleb Natapov
2013-10-31T07:48:11Z

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. 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.