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Subject: Re: [PATCH 1/2] x86,kvm: move qemu/guest FPU switching out to
 vcpu_run
From: Rik van Riel <riel@redhat.com>
To: "quan.xu04@gmail.com" <quan.xu04@gmail.com>, pbonzini@redhat.com
Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, david@redhat.com,
        borntraeger@de.ibm.com, tglx@linutronix.de, rkrcmar@redhat.com,
        Quan Xu <quan.xu0@gmail.com>
Date: Wed, 15 Nov 2017 09:43:01 -0500
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         <20171114215424.32214-2-riel@redhat.com>
         <2e83dd3f-8f03-03ac-45e0-f2fae1d17013@gmail.com>
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On Wed, 2017-11-15 at 14:53 +0800, quan.xu04@gmail.com wrote:
> 
> On 2017/11/15 05:54, riel@redhat.com wrote:
> > From: Rik van Riel <riel@redhat.com>
> > 
> > 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 you 
> could load fpu/xstate
>    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?