Re: [PATCH 1/2] x86,kvm: move qemu/guest FPU switching out to vcpu_run

[Rik van Riel]

On Wed, 2017-11-15 at 14:53 +0800, quan.xu04@xxxxxxxxx wrote:
> 
> On 2017/11/15 05:54, riel@xxxxxxxxxx wrote:
> > From: Rik van Riel <riel@xxxxxxxxxx>
> > 
> > Currently, every time a VCPU is scheduled out, the host kernel will
> > first save the guest FPU/xstate context, then load the qemu
> > userspace
> > FPU context, only to then immediately save the qemu userspace FPU
> > context back to memory. When scheduling in a VCPU, the same
> > extraneous
> > FPU loads and saves are done.
> 
> Rik, be careful with VM migration. with you patch, I don't think youÂ
> could load fpu/xstate
> ÂÂ context accurately after VM migration.

Can you explain why you believe that?

Getting the guest FPU or XSTATE is done under the vcpu->mutex.

This patch switches out guest and userspace FPU/XSTATE under the
vcpu->mutex, and switches it back before releasing the vcpu->mutex.

By the time a KVM_GET_FPU has obtained the vcpu->mutex, the guest
FPU state will be in vcpu->arch.guest_fpu.state, where you expect
it to be.

What am I missing?