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=-15.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,UNPARSEABLE_RELAY, USER_AGENT_SANE_1 autolearn=unavailable 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 E8DA3C4320A for ; Tue, 10 Aug 2021 10:30:27 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D297160E97 for ; Tue, 10 Aug 2021 10:30:27 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S238605AbhHJKas (ORCPT ); Tue, 10 Aug 2021 06:30:48 -0400 Received: from out30-132.freemail.mail.aliyun.com ([115.124.30.132]:36890 "EHLO out30-132.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231688AbhHJKar (ORCPT ); Tue, 10 Aug 2021 06:30:47 -0400 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R161e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e04395;MF=laijs@linux.alibaba.com;NM=1;PH=DS;RN=14;SR=0;TI=SMTPD_---0UiadueK_1628591422; Received: from C02XQCBJJG5H.local(mailfrom:laijs@linux.alibaba.com fp:SMTPD_---0UiadueK_1628591422) by smtp.aliyun-inc.com(127.0.0.1); Tue, 10 Aug 2021 18:30:23 +0800 Subject: Re: [PATCH V2 2/3] KVM: X86: Set the hardware DR6 only when KVM_DEBUGREG_WONT_EXIT To: Paolo Bonzini , Lai Jiangshan , linux-kernel@vger.kernel.org Cc: Sean Christopherson , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , Thomas Gleixner , Ingo Molnar , Borislav Petkov , x86@kernel.org, "H. Peter Anvin" , kvm@vger.kernel.org References: <20210809174307.145263-1-jiangshanlai@gmail.com> <20210809174307.145263-2-jiangshanlai@gmail.com> <68ed0f5c-40f1-c240-4ad1-b435568cf753@redhat.com> From: Lai Jiangshan Message-ID: <45fef019-8bd9-2acb-bd53-1243a8a07c4e@linux.alibaba.com> Date: Tue, 10 Aug 2021 18:30:21 +0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: <68ed0f5c-40f1-c240-4ad1-b435568cf753@redhat.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2021/8/10 18:07, Paolo Bonzini wrote: > On 09/08/21 19:43, Lai Jiangshan wrote: >> From: Lai Jiangshan >> >> Commit c77fb5fe6f03 ("KVM: x86: Allow the guest to run with dirty debug >> registers") allows the guest accessing to DRs without exiting when >> KVM_DEBUGREG_WONT_EXIT and we need to ensure that they are synchronized >> on entry to the guest---including DR6 that was not synced before the commit. >> >> But the commit sets the hardware DR6 not only when KVM_DEBUGREG_WONT_EXIT, >> but also when KVM_DEBUGREG_BP_ENABLED.  The second case is unnecessary >> and just leads to a more case which leaks stale DR6 to the host which has >> to be resolved by unconditionally reseting DR6 in kvm_arch_vcpu_put(). >> >> We'd better to set the hardware DR6 only when KVM_DEBUGREG_WONT_EXIT, >> so that we can fine-grain control the cases when we need to reset it >> which is done in later patch. >> >> Signed-off-by: Lai Jiangshan >> --- >>   arch/x86/kvm/x86.c | 4 +++- >>   1 file changed, 3 insertions(+), 1 deletion(-) >> >> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c >> index ad47a09ce307..d2aa49722064 100644 >> --- a/arch/x86/kvm/x86.c >> +++ b/arch/x86/kvm/x86.c >> @@ -9598,7 +9598,9 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu) >>           set_debugreg(vcpu->arch.eff_db[1], 1); >>           set_debugreg(vcpu->arch.eff_db[2], 2); >>           set_debugreg(vcpu->arch.eff_db[3], 3); >> -        set_debugreg(vcpu->arch.dr6, 6); >> +        /* When KVM_DEBUGREG_WONT_EXIT, dr6 is accessible in guest. */ >> +        if (vcpu->arch.switch_db_regs & KVM_DEBUGREG_WONT_EXIT) >> +            set_debugreg(vcpu->arch.dr6, 6); >>       } else if (unlikely(hw_breakpoint_active())) { >>           set_debugreg(0, 7); >>       } >> > > Even better, this should be moved to vmx.c's vcpu_enter_guest.  This > matches the handling in svm.c: > >         /* >          * Run with all-zero DR6 unless needed, so that we can get the exact cause >          * of a #DB. >          */ >         if (unlikely(vcpu->arch.switch_db_regs & KVM_DEBUGREG_WONT_EXIT)) >                 svm_set_dr6(svm, vcpu->arch.dr6); >         else >                 svm_set_dr6(svm, DR6_ACTIVE_LOW); > > That is, > >     KVM: X86: Set the hardware DR6 only when KVM_DEBUGREG_WONT_EXIT >     Commit c77fb5fe6f03 ("KVM: x86: Allow the guest to run with dirty debug >     registers") allows the guest accessing to DRs without exiting when >     KVM_DEBUGREG_WONT_EXIT and we need to ensure that they are synchronized >     on entry to the guest---including DR6 that was not synced before the commit. >     But the commit sets the hardware DR6 not only when KVM_DEBUGREG_WONT_EXIT, >     but also when KVM_DEBUGREG_BP_ENABLED.  The second case is unnecessary >     and just leads to a more case which leaks stale DR6 to the host which has >     to be resolved by unconditionally reseting DR6 in kvm_arch_vcpu_put(). >     Even if KVM_DEBUGREG_WONT_EXIT, however, setting the host DR6 only matters >     on VMX because SVM always uses the DR6 value from the VMCB.  So move this >     line to vmx.c and make it conditional on KVM_DEBUGREG_WONT_EXIT. >     Signed-off-by: Paolo Bonzini > > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c > index ae8e62df16dd..21a3ef3012cf 100644 > --- a/arch/x86/kvm/vmx/vmx.c > +++ b/arch/x86/kvm/vmx/vmx.c > @@ -6625,6 +6625,10 @@ static fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu) >          vmx->loaded_vmcs->host_state.cr4 = cr4; >      } > > +    /* When KVM_DEBUGREG_WONT_EXIT, dr6 is accessible in guest. */ > +    if (vcpu->arch.switch_db_regs & KVM_DEBUGREG_WONT_EXIT) > +        set_debugreg(vcpu->arch.dr6, 6); I also noticed the related code in svm.c, but I refrained myself to add a new branch in vmx_vcpu_run(). But after I see you put the code of resetting dr6 in vmx_sync_dirty_debug_regs(), the whole solution is much clean and better. And if any chance you are also concern about the additional branch, could you add a new callback to set dr6 and call the callback from x86.c when KVM_DEBUGREG_WONT_EXIT. The possible implementation of the callback: for vmx: set_debugreg(vcpu->arch.dr6, 6); for svm: svm_set_dr6(svm, vcpu->arch.dr6); and always do svm_set_dr6(svm, DR6_ACTIVE_LOW); at the end of the svm_handle_exit(). Thanks Lai > + >      /* When single-stepping over STI and MOV SS, we must clear the >       * corresponding interruptibility bits in the guest state. Otherwise >       * vmentry fails as it then expects bit 14 (BS) in pending debug > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index a111899ab2b4..fbc536b21585 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -9597,7 +9597,6 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu) >          set_debugreg(vcpu->arch.eff_db[1], 1); >          set_debugreg(vcpu->arch.eff_db[2], 2); >          set_debugreg(vcpu->arch.eff_db[3], 3); > -        set_debugreg(vcpu->arch.dr6, 6); >      } else if (unlikely(hw_breakpoint_active())) { >          set_debugreg(0, 7); >      } > > Paolo