On Tue, Nov 05, 2013 at 12:18:57PM +0200, Michael S. Tsirkin wrote: > On Mon, Nov 04, 2013 at 10:44:43PM +0200, Gleb Natapov wrote: > > On Mon, Nov 04, 2013 at 10:33:57PM +0200, Michael S. Tsirkin wrote: > > > On Mon, Nov 04, 2013 at 10:13:39PM +0200, Gleb Natapov wrote: > > > > On Mon, Nov 04, 2013 at 10:11:33PM +0200, Michael S. Tsirkin wrote: > > > > > 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. > > > > > > > > > So what? You do not read what I wrote. > > > > > > > > > It's simple: if I revert 8bf00a529967dafbbb210 then exit latency > > > is reduced. > > > You seem to tell me it should be a nop, but in practice it isn't. > > > > > > > No, if you read below I am saying that it looks like you are claiming that > > generic msr switch mechanism is faster and I am not buying that. If you > > believe this to be the case ask Intel for explanation. Your claim about > > "not running guest under perf" is even stranger since in this case no msr > > switch should happen regardless of the aforementioned commit (unless guest > > or host runs nmi watchdog, but then switch will happen no matter if perf > > is running, so again not running guest under perf" does not make sense). > > So, in short, you do not really know where the slow down is coming > > from. > > That's true. > Then dig dipper. > > My guess is that it comes from the fact that we unconditionally > > call clear_atomic_switch_msr() in atomic_switch_perf_msrs(), but then > > fix that instead of reverting the patch. > > We can try, but reverting is much simpler, it removes code instead of > adding code. Well, this is such absurd statement I do not really know what to say :) > Do you know which workload is actually improved by > 8bf00a529967dafbbb210? > Switching PERF_GLOBAL_CTRL when it needs to be switched. -- Gleb. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
