Re: [PATCH 0/7] KVM: x86: APX reg prep work

[Andrew Cooper <andrew.cooper3@citrix.com>]

> Have you measured performance/latency overhead if KVM goes straight to context
> switching R16-R31 at entry/exit?  With PUSH2/POP2, it's "only" 8 more instructions
> on each side.
>
> If the overhead is in the noise, I'd be very strongly inclined to say KVM should
> swap at entry/exit regardless of kernel behavior so that we don't have to special
> case accesses on the back end.

I tried raising this point at plumbers but I don't think it came through
well.

You can't unconditionally use PUSH2/POP2 in the VMExit, because at that
point in time it's the guest's XCR0 in context.  If the guest has APX
disabled, PUSH2 in the VMExit path will #UD.

You either need two VMExit handlers, one APX and one non-APX and choose
based on the guest XCR0 value, or you need a branch prior to regaining
speculative safety, or you need to save/restore XCR0 as the first
action.  It's horrible any way you look at it.

I've asked both Intel and AMD for changes to VT-x/SVM to have a proper
host/guest split of XCR0 which hardware manages on entry/exit.  It's the
only viable option in my opinion, but it's still an unknown period of
time away and not going to exist in the first APX-capable hardware.

~Andrew