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 13:18:34 +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 13:12, Quan Xu wrote:
> However it seems there is still some gap..
> 
> as Rik said, "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."

By "guest FPU state" Rik means "guest FPU with host PKRU".

Guest PKRU is always stored in vcpu->arch.pkru rather than in the guest
FPU state, so guest PKRU will never be in current->thread.fpu.state either.

KVM_GET_XSAVE will the guest FPU state with vcpu->arch.pkru and
migration will work properly.

Thanks,

Paolo

> after preempt_enable() in vcpu_enter_guest(), the vcpu thread is
> scheduled out,
> in kvm_x86_ops->run, the PKRU has been restored with host_pkru (IF
> guest_pkru != host_pkru)..
> instead of guest_pkru..
> 
> then the PKRU is host_pkru, how to save guest_pkru current->thread.fpu?
> 
> as mentioned, all this _assumes_ that the pkru is in
> current->thread.fpu.state as well.
> 
> 
> thanks,
> 
> Quan
> Alibaba Cloud
> 
>> 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..
>>
>