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

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Date:   Wed, 15 Nov 2017 23:21:34 -0500
From:   Rik van Riel <riel@...hat.com>
To:     Quan Xu <quan.xu0@...il.com>,
        "quan.xu04@...il.com" <quan.xu04@...il.com>, pbonzini@...hat.com
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        david@...hat.com, borntraeger@...ibm.com, tglx@...utronix.de,
        rkrcmar@...hat.com
Subject: Re: [PATCH 1/2] x86,kvm: move qemu/guest FPU switching out to
 vcpu_run

On Thu, 2017-11-16 at 10:50 +0800, Quan Xu wrote:
> 
> On 2017-11-15 22:43, Rik van Riel wrote:
> > Can you explain why you believe that?
> 
> for example, a vcpu thread is running in kvm mode under cretical
> condition to stop. QEMU send an IPI to cause a VM-exit to happen
> immediately, and this IPI doesn't make vcpu return to QEMU. IIUC
> this vcpu thread will still continue to run in kvm mode when is
> waked up at targer machine. with your patch, I don't see a chance
> to load guest FPU or XSTATE, until return to QEMU and run kvm mode
> again.
> 
> then the FPU or XSTATE status is inconsistent for a small window,
> what's 
> even
> worse is that the vcpu is running.
> 
> Did I misunderstand?

At context switch time, the context switch code will save
the guest FPU state to current->thread.fpu when the
VCPU thread is scheduled out.

When the VCPU thread is scheduled back in, the context
switch code will restore current->thread.fpu to the FPU
registers.

The VCPU thread will never run with anything else than
the guest FPU state, while inside the KVM_RUN code.