On 2015-04-07 07:43, Jan Kiszka wrote: > On 2015-04-05 19:12, Valentine Sinitsyn wrote: >> Hi Jan, >> >> On 05.04.2015 13:31, Jan Kiszka wrote: >>> studying the VM exit logic of Jailhouse, I was wondering when AMD's >>> vmload/vmsave can be avoided. Jailhouse as well as KVM currently use >>> these instructions unconditionally. However, I think both only need >>> GS.base, i.e. the per-cpu base address, to be saved and restored if no >>> user space exit or no CPU migration is involved (both is always true for >>> Jailhouse). Xen avoids vmload/vmsave on lightweight exits but it also >>> still uses rsp-based per-cpu variables. >>> >>> So the question boils down to what is generally faster: >>> >>> A) vmload >>> vmrun >>> vmsave >>> >>> B) wrmsrl(MSR_GS_BASE, guest_gs_base) >>> vmrun >>> rdmsrl(MSR_GS_BASE, guest_gs_base) >>> >>> Of course, KVM also has to take into account that heavyweight exits >>> still require vmload/vmsave, thus become more expensive with B) due to >>> the additional MSR accesses. >>> >>> Any thoughts or results of previous experiments? >> That's a good question, I also thought about it when I was finalizing >> Jailhouse AMD port. I tried "lightweight exits" with apic-demo but it >> didn't seem to affect the latency in any noticeable way. That's why I >> decided not to push the patch (in fact, I was even unable to find it now). >> >> Note however that how AMD chips store host state during VM switches are >> implementation-specific. I did my quick experiments on one CPU only, so >> your mileage may vary. >> >> Regarding your question, I feel B will be faster anyways but again I'm >> afraid that the gain could be within statistical error of the experiment. > > It is, at least 160 cycles with hot caches on an AMD A6-5200 APU, more > towards 600 if they are colder (added some usleep to each loop in the test). > > I've tested via vmmcall from guest userspace under Jailhouse. KVM should > be adjustable in a similar way. Attached the benchmark, patch will be in > the Jailhouse next branch soon. We need to check more CPU types, though. Avi, I found some preparatory patches of yours from 2010 [1]. Do you happen to remember if it was never completed for a technical reason? Joel, can you comment on the benefit of variant B) for the various AMD CPUs? Is it always positive? Thanks, Jan [1] http://thread.gmane.org/gmane.comp.emulators.kvm.devel/61455 -- Siemens AG, Corporate Technology, CT RTC ITP SES-DE Corporate Competence Center Embedded Linux -- 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
