From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 6625AC76188 for ; Fri, 19 Jul 2019 11:09:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 44CA721849 for ; Fri, 19 Jul 2019 11:09:09 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727586AbfGSLJH (ORCPT ); Fri, 19 Jul 2019 07:09:07 -0400 Received: from mail-wm1-f67.google.com ([209.85.128.67]:50447 "EHLO mail-wm1-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726075AbfGSLJH (ORCPT ); Fri, 19 Jul 2019 07:09:07 -0400 Received: by mail-wm1-f67.google.com with SMTP id v15so28426914wml.0 for ; Fri, 19 Jul 2019 04:09:05 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:subject:to:cc:references:from:openpgp:message-id :date:user-agent:mime-version:in-reply-to:content-language :content-transfer-encoding; bh=jZTpt124xkvIC199mqq/Y1JoK7GIwu0z9IqI5bP/sDw=; b=Xr3k9i5F5DJu1/S7/rCxV4AX6IUw0oz8z1WAHK4jJuBlI22iSiuyo3c2j88U82TD8C m0WMYwrzxenCXAT65hUgUKZpPxX2UXyil+6x1xli09trz91/g22Ojk67n4MY3y3DYyG6 ovOsMqowWafm1te2BdSU9g8N+FTvnMqpj4oXvUHG+dbv8f+eBhzmq9fZxNUAqms2diJC jIXW1HpdEfRNZrBp8SH5Z++Fiw+uga/mHk47jIzZVFN01EcPjY2P1pEB3N18i2PqR4rw RZiWrUKkclhuBRzcgf+PGKfSXE/yDaxHnr9hE8YyQXm19f3UqlV6K5BwakPTc+SySvVh gkOg== X-Gm-Message-State: APjAAAWywainqqz39MaNq+WYwALdiHY3T995Bn+u2uT5WBnZPTwAXCR2 nwAoUi7+RP+lidFORHKkM67zCQ== X-Google-Smtp-Source: APXvYqy8J64o/W1vhWCroMIOpTpGntL1yVyaLhpT/aa7pg1hpD5LFTyt3PBsqrwCwNryxxXKXqVQVA== X-Received: by 2002:a7b:c215:: with SMTP id x21mr47986773wmi.38.1563534544929; Fri, 19 Jul 2019 04:09:04 -0700 (PDT) Received: from ?IPv6:2001:b07:6468:f312:8501:6b03:f18c:74f8? ([2001:b07:6468:f312:8501:6b03:f18c:74f8]) by smtp.gmail.com with ESMTPSA id l2sm20503956wmj.4.2019.07.19.04.09.03 (version=TLS1_3 cipher=AEAD-AES128-GCM-SHA256 bits=128/128); Fri, 19 Jul 2019 04:09:04 -0700 (PDT) Subject: Re: [5.2 regression] x86/fpu changes cause crashes in KVM guest To: Wanpeng Li , Thomas Lambertz Cc: Sebastian Andrzej Siewior , Rik van Riel , Dave Hansen , Thomas Gleixner , Ingo Molnar , Borislav Petkov , "H. Peter Anvin" , the arch/x86 maintainers , LKML , Radim Krcmar , kvm , Peter Zijlstra , Marc Orr , Dave Hansen References: <217248af-e980-9cb0-ff0d-9773413b9d38@thomaslambertz.de> From: Paolo Bonzini Openpgp: preference=signencrypt Message-ID: <3ae96202-a121-70a9-fe00-4b5bb4970242@redhat.com> Date: Fri, 19 Jul 2019 13:09:03 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 19/07/19 10:59, Wanpeng Li wrote: > https://lkml.org/lkml/2017/11/14/891, "The scheduler will save the > guest fpu context when a vCPU thread is preempted, and restore it when > it is scheduled back in." But I can't find any scheduler codes do > this. That's because applying commit 240c35a37 was completely wrong. The idea before commit 240c35a37 was that you have the following FPU states: userspace (QEMU) guest --------------------------------------------------------------------------- processor vcpu->arch.guest_fpu >>> KVM_RUN: kvm_load_guest_fpu vcpu->arch.user_fpu processor >>> preempt out vcpu->arch.user_fpu current->thread.fpu >>> preempt in vcpu->arch.user_fpu processor >>> back to userspace >>> kvm_put_guest_fpu processor vcpu->arch.guest_fpu --------------------------------------------------------------------------- After removing user_fpu, QEMU's FPU state is destroyed when KVM_RUN is preempted. So that's already messed up (I'll send a revert), and given the diagram above your patch makes total sense. With the new lazy model we want to hook into kvm_vcpu_arch_load and get the state back to the processor from current->thread.fpu, and indeed switch_fpu_return is essentially copy_kernel_to_fpregs(¤t->thread. fpu->state). However I would keep the fpregs_assert_state_consistent in kvm_arch_vcpu_load, and also WARN_ON_ONCE(test_thread_flag(TIF_NEED_FPU_LOAD)) in vcpu_enter_guest. Paolo