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

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Date:   Thu, 16 Nov 2017 11:21:39 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Quan Xu <quan.xu0@...il.com>, 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
Subject: Re: [PATCH 1/2] x86,kvm: move qemu/guest FPU switching out to
 vcpu_run

On 16/11/2017 06:06, Quan Xu wrote:
> when vcpu thread is scheduled out, the pkru value in
> current->thread.fpu.state may be the host pkru value, instead of
> guest pkru value (of course, this _assumes_ that the pkru is in
> current->thread.fpu.state as well). in this way, the pkru may be a
> coner case.

Rik may correct me, but I think this is not possible.  Preemption is
disabled all the time while PKRU = guest_pkru (which is only during
vmx_vcpu_run).

Context switching will only happen in vcpu_enter_guest() after
preempt_enable() for a preemptible kernel, or in vcpu_run via
cond_resched() for a non-preemptible kernel.

Thanks,

Paolo

> 
> VM migration again, in case,
>            source_host_pkru_value != guest_pkru_value,
>            target_host_pkru_value == guest_pkru_value..
> 
> the pkru status would be inconsistent..