From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: Sean Christopherson <seanjc@google.com>
Cc: kvm@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
Wanpeng Li <wanpengli@tencent.com>,
Jim Mattson <jmattson@google.com>,
Maxim Levitsky <mlevitsk@redhat.com>,
Michael Kelley <mikelley@microsoft.com>,
linux-hyperv@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4 6/6] KVM: selftests: Test Hyper-V invariant TSC control
Date: Wed, 12 Oct 2022 14:40:52 +0200 [thread overview]
Message-ID: <87v8op6wq3.fsf@ovpn-194-196.brq.redhat.com> (raw)
In-Reply-To: <Y0XGuk4vwJBTU9oN@google.com>
Sean Christopherson <seanjc@google.com> 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
next prev parent reply other threads:[~2022-10-12 12:41 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-09-22 14:36 [PATCH v4 0/6] KVM: x86: Hyper-V invariant TSC control feature Vitaly Kuznetsov
2022-09-22 14:36 ` [PATCH v4 1/6] x86/hyperv: Add HV_INVARIANT_TSC_EXPOSED define Vitaly Kuznetsov
2022-09-22 22:07 ` Michael Kelley (LINUX)
2022-09-22 14:36 ` [PATCH v4 2/6] KVM: x86: Introduce CPUID_8000_0007_EDX 'scattered' leaf Vitaly Kuznetsov
2022-09-22 17:09 ` Jim Mattson
2022-09-22 17:53 ` Sean Christopherson
2022-09-22 17:55 ` Jim Mattson
2022-10-11 19:15 ` Sean Christopherson
2022-09-22 14:36 ` [PATCH v4 3/6] KVM: x86: Hyper-V invariant TSC control Vitaly Kuznetsov
2022-09-22 14:36 ` [PATCH v4 4/6] KVM: selftests: Rename 'msr->availble' to 'msr->fault_exepected' in hyperv_features test Vitaly Kuznetsov
2022-10-11 19:18 ` Sean Christopherson
2022-09-22 14:36 ` [PATCH v4 5/6] KVM: selftests: Convert hyperv_features test to using KVM_X86_CPU_FEATURE() Vitaly Kuznetsov
2022-09-22 14:36 ` [PATCH v4 6/6] KVM: selftests: Test Hyper-V invariant TSC control Vitaly Kuznetsov
2022-10-11 19:40 ` Sean Christopherson
2022-10-12 12:40 ` Vitaly Kuznetsov [this message]
2022-10-12 16:52 ` Sean Christopherson
2022-10-13 9:16 ` Vitaly Kuznetsov
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