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. > > > 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. > > > -- > > > 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 > > -- > 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
