Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH 1/2] x86,kvm: move qemu/guest FPU switching out to
 vcpu_run
To:     Paolo Bonzini <pbonzini@redhat.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>
From:   Quan Xu <quan.xu0@gmail.com>
Message-ID: <afe14c82-1038-72f4-1471-50a5db5c2235@gmail.com>
Date:   Thu, 16 Nov 2017 20:12:58 +0800
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101
 Thunderbird/52.4.0
MIME-Version: 1.0
In-Reply-To: <9ba2d4c1-0781-419e-ddeb-77e0494583d2@redhat.com>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 8bit
Content-Language: en-US
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


On 2017-11-16 18:21, Paolo Bonzini wrote:
> 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).

refer to the following code,

vcpu_enter_guest()
{
     ...
     preempt_disable()
        ...
        kvm_x86_ops->run(vcpu);  (actually it is vmx_vcpu_run())
        ...
     ...
     preempt_enable();
}


when preempt_disable before to run kvm_x86_ops->run..
in kvm_x86_ops->run, the pkru is restored with host_pkru (IF guest_pkru 
!= host_pkru).
all this happened under preempt_disable().

it's not true that preemtion is disable all the time while PKRU  = 
guest_pkru..


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."

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..
>


From 1584225337503196489@xxx Thu Nov 16 12:23:35 +0000 2017
X-GM-THRID: 1584017174910331026
X-Gmail-Labels: Inbox,Category Forums,HistoricalUnread