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([2001:b07:6468:f312:5e2c:eb9a:a8b6:fd3e]) by smtp.gmail.com with ESMTPSA id j13sm28646969wrt.29.2021.03.29.02.39.52 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 29 Mar 2021 02:39:53 -0700 (PDT) Subject: Re: [PATCH] KVM: x86/vPMU: Forbid writing to MSR_F15H_PERF MSRs when guest doesn't have X86_FEATURE_PERFCTR_CORE To: Vitaly Kuznetsov , kvm@vger.kernel.org Cc: Sean Christopherson , Wanpeng Li , Jim Mattson , Wei Huang , Joerg Roedel , linux-kernel@vger.kernel.org References: <20210323084515.1346540-1-vkuznets@redhat.com> <874kgubau4.fsf@vitty.brq.redhat.com> From: Paolo Bonzini Message-ID: Date: Mon, 29 Mar 2021 11:39:51 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.0 MIME-Version: 1.0 In-Reply-To: <874kgubau4.fsf@vitty.brq.redhat.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On 29/03/21 10:52, Vitaly Kuznetsov wrote: > Paolo Bonzini writes: > >> On 23/03/21 09:45, Vitaly Kuznetsov wrote: >>> MSR_F15H_PERF_CTL0-5, MSR_F15H_PERF_CTR0-5 MSRs are only available when >>> X86_FEATURE_PERFCTR_CORE CPUID bit was exposed to the guest. KVM, however, >>> allows these MSRs unconditionally because kvm_pmu_is_valid_msr() -> >>> amd_msr_idx_to_pmc() check always passes and because kvm_pmu_set_msr() -> >>> amd_pmu_set_msr() doesn't fail. >>> >>> In case of a counter (CTRn), no big harm is done as we only increase >>> internal PMC's value but in case of an eventsel (CTLn), we go deep into >>> perf internals with a non-existing counter. >>> >>> Note, kvm_get_msr_common() just returns '0' when these MSRs don't exist >>> and this also seems to contradict architectural behavior which is #GP >>> (I did check one old Opteron host) but changing this status quo is a bit >>> scarier. >> >> Hmm, since these do have a cpuid bit it may not be that scary. > > Well, if you're not scared I can send a patch) Go ahead. Paolo