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 13:06:18 +0800
From:   Quan Xu <quan.xu0@...il.com>
To:     Rik van Riel <riel@...hat.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 2017-11-16 12:21, Rik van Riel wrote:
> 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.


good catch!
Also as your comment, PKRU is switched out separately at
VMENTER and VMEXIT time, but with a lots of IF conditions..

the pkru may be restored with host pkru after VMEXIT. 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.

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..



Quan
Alibaba Cloud

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