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[91.219.240.2]) by smtp.gmail.com with ESMTPSA id g17-20020a05600c001100b003c6bbe910fdsm2079979wmc.9.2022.10.12.05.40.53 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 12 Oct 2022 05:40:53 -0700 (PDT) From: Vitaly Kuznetsov To: Sean Christopherson Cc: kvm@vger.kernel.org, Paolo Bonzini , Wanpeng Li , Jim Mattson , Maxim Levitsky , Michael Kelley , linux-hyperv@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v4 6/6] KVM: selftests: Test Hyper-V invariant TSC control In-Reply-To: References: <20220922143655.3721218-1-vkuznets@redhat.com> <20220922143655.3721218-7-vkuznets@redhat.com> Date: Wed, 12 Oct 2022 14:40:52 +0200 Message-ID: <87v8op6wq3.fsf@ovpn-194-196.brq.redhat.com> MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Sean Christopherson writes: > On Thu, Sep 22, 2022, Vitaly Kuznetsov wrote: >> diff --git a/tools/testing/selftests/kvm/x86_64/hyperv_features.c b/tools/testing/selftests/kvm/x86_64/hyperv_features.c >> index d4bd18bc580d..18b44450dfb8 100644 >> --- a/tools/testing/selftests/kvm/x86_64/hyperv_features.c >> +++ b/tools/testing/selftests/kvm/x86_64/hyperv_features.c >> @@ -46,20 +46,33 @@ struct hcall_data { >> >> static void guest_msr(struct msr_data *msr) >> { >> - uint64_t ignored; >> + uint64_t msr_val = 0; >> uint8_t vector; >> >> GUEST_ASSERT(msr->idx); >> >> - if (!msr->write) >> - vector = rdmsr_safe(msr->idx, &ignored); >> - else >> + if (!msr->write) { >> + vector = rdmsr_safe(msr->idx, &msr_val); > > This is subtly going to do weird things if the RDMSR faults. rdmsr_safe() > overwrites @val with whatever happens to be in EDX:EAX if the RDMSR faults, i.e. > this may yield garbage instead of '0'. Arguably rdmsr_safe() is a bad API, but > at the same time the caller really shouldn't consume the result if RDMSR faults > (though aligning with the kernel is also valuable). > > Aha! Idea. Assuming none of the MSRs are write-only, what about adding a prep > patch to rework this code so that it verifies RDMSR returns what was written when > a fault didn't occur. > There is at least one read-only MSR which comes to mind: HV_X64_MSR_EOI. Also, some of the MSRs don't preserve the written value, e.g. HV_X64_MSR_RESET which always reads as '0'. I do, however, like the additional check that RDMSR returns what was written to the MSR, we will just need an additional flag in 'struct msr_data' ('check_written_value' maybe?). -- Vitaly