From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751696AbdKPFGi (ORCPT ); Thu, 16 Nov 2017 00:06:38 -0500 Received: from mail-ot0-f175.google.com ([74.125.82.175]:52810 "EHLO mail-ot0-f175.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750981AbdKPFG1 (ORCPT ); Thu, 16 Nov 2017 00:06:27 -0500 X-Google-Smtp-Source: AGs4zMbsvbt1qkR9HoysZTd4DuH3UwmNRDC0ZJzYtVijCxcrUCyWb1SsfDaDt5Ejllp0KxcOSvV4ig== Subject: Re: [PATCH 1/2] x86,kvm: move qemu/guest FPU switching out to vcpu_run To: Rik van Riel , "quan.xu04@gmail.com" , pbonzini@redhat.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> From: Quan Xu Message-ID: <0fc15536-0b34-51b6-cc14-00989825b0bc@gmail.com> Date: Thu, 16 Nov 2017 13:06:18 +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: <1510806094.21121.303.camel@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 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. >