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Subject: Re: [PATCH 2/2] KVM: x86: introduce KVM_X86_QUIRK_TSC_HOST_ACCESS
To:     Maxim Levitsky <mlevitsk@redhat.com>, kvm@vger.kernel.org
Cc:     Oliver Upton <oupton@google.com>, Ingo Molnar <mingo@redhat.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        open list <linux-kernel@vger.kernel.org>,
        Marcelo Tosatti <mtosatti@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Wanpeng Li <wanpengli@tencent.com>,
        Borislav Petkov <bp@alien8.de>,
        Jim Mattson <jmattson@google.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        Joerg Roedel <joro@8bytes.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        Vitaly Kuznetsov <vkuznets@redhat.com>
References: <20201130133559.233242-1-mlevitsk@redhat.com>
 <20201130133559.233242-3-mlevitsk@redhat.com>
 <c093973e-c8da-4d09-11f2-61cc0918f55f@redhat.com>
 <638a2919cf7c11c55108776beecafdd8e2da2995.camel@redhat.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <5e77e912-893b-0c8f-a9a6-b43eaee24ed3@redhat.com>
Date:   Mon, 30 Nov 2020 15:15:11 +0100
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On 30/11/20 15:11, Maxim Levitsky wrote:
> On Mon, 2020-11-30 at 14:54 +0100, Paolo Bonzini wrote:
>> On 30/11/20 14:35, Maxim Levitsky wrote:
>>> This quirk reflects the fact that we currently treat MSR_IA32_TSC
>>> and MSR_TSC_ADJUST access by the host (e.g qemu) in a way that is different
>>> compared to an access from the guest.
>>>
>>> For host's MSR_IA32_TSC read we currently always return L1 TSC value, and for
>>> host's write we do the tsc synchronization.
>>>
>>> For host's MSR_TSC_ADJUST write, we don't make the tsc 'jump' as we should
>>> for this msr.
>>>
>>> When the hypervisor uses the new TSC GET/SET state ioctls, all of this is no
>>> longer needed, thus leave this enabled only with a quirk
>>> which the hypervisor can disable.
>>>
>>> Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
>>> Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
>>
>> This needs to be covered by a variant of the existing selftests testcase
>> (running the same guest code, but different host code of course).
> Do you think that the test should go to the kernel's kvm unit tests,
> or to kvm-unit-tests project?

The latter already has x86_64/tsc_msrs_test.c (which I created in 
preparation for this exact change :)).

Paolo