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[91.219.240.2]) by smtp.gmail.com with ESMTPSA id h18-20020a05600c351200b003a31df6af2esm19012657wmq.1.2022.08.01.01.16.53 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 01 Aug 2022 01:16:53 -0700 (PDT) From: Vitaly Kuznetsov To: Paolo Bonzini , Sean Christopherson Cc: kvm@vger.kernel.org, Anirudh Rayabharam , Wanpeng Li , Jim Mattson , Maxim Levitsky , linux-hyperv@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v4 01/25] KVM: x86: hyper-v: Expose access to debug MSRs in the partition privilege flags In-Reply-To: <0741719d-17b4-96fe-1ee8-5f22cf3e255b@redhat.com> References: <20220714091327.1085353-1-vkuznets@redhat.com> <20220714091327.1085353-2-vkuznets@redhat.com> <0741719d-17b4-96fe-1ee8-5f22cf3e255b@redhat.com> Date: Mon, 01 Aug 2022 10:16:52 +0200 Message-ID: <87a68o2xaj.fsf@redhat.com> MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: linux-hyperv@vger.kernel.org Paolo Bonzini writes: > On 7/21/22 23:43, Sean Christopherson wrote: >> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c >> index c284a605e453..ca91547034e4 100644 >> --- a/arch/x86/kvm/hyperv.c >> +++ b/arch/x86/kvm/hyperv.c >> @@ -1282,7 +1282,7 @@ static bool hv_check_msr_access(struct kvm_vcpu_hv *hv_vcpu, u32 msr) >> case HV_X64_MSR_SYNDBG_OPTIONS: >> case HV_X64_MSR_SYNDBG_CONTROL ... HV_X64_MSR_SYNDBG_PENDING_BUFFER: >> return hv_vcpu->cpuid_cache.features_edx & >> - HV_FEATURE_DEBUG_MSRS_AVAILABLE; >> + HV_ACCESS_DEBUG_MSRS; >> default: >> break; >> } >> > > Yes, and this will need some kind of hack in QEMU to expose both CPUID > bits. Fortunately hv-syndbg shouldn't be in much use in the wild, so I > think we can avoid quirks etc. Properly behaving VMM should always expose both bits. I'm not sure what would it mean if only 'access' bit is present: you can try accessing the missing feature but you get #GP anyway most likely. When the feature is available but 'access' bit is not set -- the result is also #GP. In case we really want to support this behavior in KVM we should probably check *both* bits in hv_check_msr_access() but I don't really see a use-case. I've lazily kept HV_FEATURE_DEBUG_MSRS_AVAILABLE here just to be QEMU compatible. -- Vitaly