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Subject: Re: [PATCH 1/2] x86,kvm: move qemu/guest FPU switching out to
 vcpu_run
To:     Quan Xu <quan.xu0@gmail.com>, Rik van Riel <riel@redhat.com>,
        "quan.xu04@gmail.com" <quan.xu04@gmail.com>
Cc:     kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        david@redhat.com, borntraeger@de.ibm.com, tglx@linutronix.de,
        rkrcmar@redhat.com
References: <20171114215424.32214-1-riel@redhat.com>
 <20171114215424.32214-2-riel@redhat.com>
 <2e83dd3f-8f03-03ac-45e0-f2fae1d17013@gmail.com>
 <1510756981.21121.291.camel@redhat.com>
 <c331a215-2b7c-3c79-91ce-2b10b1882d36@gmail.com>
 <1510806094.21121.303.camel@redhat.com>
 <0fc15536-0b34-51b6-cc14-00989825b0bc@gmail.com>
 <9ba2d4c1-0781-419e-ddeb-77e0494583d2@redhat.com>
 <afe14c82-1038-72f4-1471-50a5db5c2235@gmail.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <00231a24-f1a2-324e-1c32-0a5a7a3995f1@redhat.com>
Date:   Thu, 16 Nov 2017 13:18:34 +0100
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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..
>>
> 


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