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[34.168.104.7]) by smtp.gmail.com with ESMTPSA id w196-20020a6282cd000000b00553d573222fsm67015pfd.199.2022.10.12.09.52.21 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 12 Oct 2022 09:52:21 -0700 (PDT) Date: Wed, 12 Oct 2022 16:52:17 +0000 From: Sean Christopherson To: Vitaly Kuznetsov 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 Message-ID: References: <20220922143655.3721218-1-vkuznets@redhat.com> <20220922143655.3721218-7-vkuznets@redhat.com> <87v8op6wq3.fsf@ovpn-194-196.brq.redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87v8op6wq3.fsf@ovpn-194-196.brq.redhat.com> Precedence: bulk List-ID: X-Mailing-List: linux-hyperv@vger.kernel.org On Wed, Oct 12, 2022, Vitaly Kuznetsov wrote: > 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. I assume s/read-only/write-only since it's EOI? > Also, some of the MSRs don't preserve the written value, > e.g. HV_X64_MSR_RESET which always reads as '0'. Hrm, that's annoying. > 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?). Rather than force the testcase to specify information that's intrinsic to the MSR, what about adding helpers to communicate the types? E.g. if (msr->write) vector = wrmsr_safe(msr->idx, msr->write_val); if (!vector && !is_write_only_msr(msr->idx)) vector = rdmsr_safe(msr->idx, &msr_val); if (msr->fault_expected) GUEST_ASSERT_2(vector == GP_VECTOR, msr->idx, vector); else GUEST_ASSERT_2(!vector, msr->idx, vector); if (is_read_zero_msr(msr->idx)) GUEST_ASSERT_2(msr_val == 0, msr_val, 0); else GUEST_ASSERT_2(msr_val == msr->write_val, msr_val, msr->write_val); I think that'd make the code a bit less magical and easier to understand for folks that aren't familiar with Hyper-V. The number of special MSRs is likely very small, so the helpers should be trivial, e.g. static bool is_write_only_msr(uint32_t msr) { return msr == HV_X64_MSR_EOI; } static bool is_read_zero_msr(uint32_t msr) { return msr == HV_X64_MSR_RESET || msr == ???; }