linux-kernel - Re: [PATCH 1/2] x86,kvm: move qemu/guest FPU switching out to vcpu

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Message-ID: <9bd489dd-942b-71a3-aa0b-34387e113485@redhat.com>
Date:   Wed, 15 Nov 2017 16:09:23 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Rik van Riel <riel@...hat.com>,
        "quan.xu04@...il.com" <quan.xu04@...il.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        david@...hat.com, borntraeger@...ibm.com, tglx@...utronix.de,
        rkrcmar@...hat.com, Quan Xu <quan.xu0@...il.com>
Subject: Re: [PATCH 1/2] x86,kvm: move qemu/guest FPU switching out to
 vcpu_run

On 15/11/2017 15:43, Rik van Riel wrote:
>> 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?

Nothing as far as I can see. :)

Paolo