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

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Message-ID: <e3cf353c-a62f-8e1c-685d-3a971605590a@gmail.com>
Date:   Thu, 16 Nov 2017 21:35:53 +0800
From:   Quan Xu <quan.xu0@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.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 2017-11-16 20:18, Paolo Bonzini wrote:
> 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".
   :( actually it is host_pkru, just with different names..

> 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.
   agreed, I fix it.. that's why I concern.. there are so much methods to
   write PKRU with host_pkru/guest_pkru..

  after migration, KVM_GET|SET_XSAVE restore the right pkru.. but we 
introduce
  another method:

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


there is still a window to restore current->thread.fpu to the FPU 
registers before enter guest mode and

preempt_disable().

on target machine, after migration, the pkru value is source_host_pkru 
in current->thread.fpu.

in case,

             source_host_pkru_value != guest_pkru_value,
             target_host_pkru_value == guest_pkru_value..

source_host_pkru_value may be restored to PKRU.. make pkru status inconsistent..


thanks

Quan
Alibaba Cloud
> 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..
>