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. -- 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
