On Tue, Nov 05, 2013 at 06:40:04PM +0200, Gleb Natapov wrote: > On Tue, Nov 05, 2013 at 06:36:41PM +0200, Michael S. Tsirkin wrote: > > On Tue, Nov 05, 2013 at 01:20:10PM +0200, Gleb Natapov wrote: > > > On Tue, Nov 05, 2013 at 12:22:49PM +0200, Gleb Natapov wrote: > > > > 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. > > > > > > > So quick and dirty patch to not needlessly write into VM_ENTRY_CONTROLS > > > when no PERF_GLOBAL_CTRL switching needed removes all the overhead. But we > > > probably need more generic code to shadow entire > > > VM_ENTRY_CONTROLS/VM_EXIT_CONTROLS. > > > > Hmm, seems to help but less than 50 cycles, seems to be > > around 20 and that kind of gain is getting hard to measure. > > > With the patch the code with and without MSR_CORE_PERF_GLOBAL_CTRL switch > is exactly same when no switching is needed (nothing is done basically), > so I doubt your measurement.
OK I'll retry when I have the time. > For me result is exactly same (it is noisy, > but noisy in the same way). > > -- > Gleb. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
