From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934623AbdKPMNO (ORCPT ); Thu, 16 Nov 2017 07:13:14 -0500 Received: from mail-oi0-f65.google.com ([209.85.218.65]:49632 "EHLO mail-oi0-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934437AbdKPMNH (ORCPT ); Thu, 16 Nov 2017 07:13:07 -0500 X-Google-Smtp-Source: AGs4zMYSpLJEckHCcxkZ/HYvovubhidpYo105hrcdzWKSsPje4gLHFGKP7K5JYD1mb+FT1L06BWZMw== Subject: Re: [PATCH 1/2] x86,kvm: move qemu/guest FPU switching out to vcpu_run To: Paolo Bonzini , Rik van Riel , "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> <1510806094.21121.303.camel@redhat.com> <0fc15536-0b34-51b6-cc14-00989825b0bc@gmail.com> <9ba2d4c1-0781-419e-ddeb-77e0494583d2@redhat.com> From: Quan Xu Message-ID: 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 List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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.. >