Re: [PATCH v2] KVM: SVM: use vmsave/vmload for saving/restoring additional host state

Paolo Bonzini
2020-12-16T15:25:11Z

On 16/12/20 16:12, Michael Roth wrote:
> It looks like it does save us ~20-30 cycles vs. vmload, but maybe not
> enough to justify the added complexity. Additionally, since we still
> need to call vmload when we exit to userspace, it ends up being a bit
> slower for this particular workload at least. So for now I'll plan on
> sticking to vmload'ing after vmexit and moving that to the asm code
> if there are no objections.

Yeah, agreed. BTW you can use "./x86/run x86/vmexit.flat" from
kvm-unit-tests to check the numbers for a wide range of vmexit paths.

Paolo

> current v2 patch, sample 1
> ioctl entry: 1204722748832
> pre-run: 1204722749408 ( +576)
> post-run: 1204722750784 (+1376)
> ioctl exit: 1204722751360 ( +576)
> total cycles: 2528
>
> current v2 patch, sample 2
> ioctl entry: 1204722754784
> pre-vmrun: 1204722755360 ( +576)
> post-vmrun: 1204722756720 (+1360)
> ioctl exit: 1204722757312 ( +592)
> total cycles 2528
>
> wrgsbase, sample 1
> ioctl entry: 1346624880336
> pre-vmrun: 1346624880912 ( +576)
> post-vmrun: 1346624882256 (+1344)
> ioctl exit: 1346624882912 ( +656)
> total cycles 2576
>
> wrgsbase, sample 2
> ioctl entry: 1346624886272
> pre-vmrun: 1346624886832 ( +560)
> post-vmrun: 1346624888176 (+1344)
> ioctl exit: 1346624888816 ( +640)
> total cycles: 2544
>