On 2015-04-07 08:10, Valentine Sinitsyn wrote: > Hi Jan, > > On 07.04.2015 10:43, 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). > Great, thanks. Could you post absolute numbers, i.e how long do A and B > take on your CPU? A is around 1910 cycles, B about 1750. Jan -- 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
