linux-kernel - Re: [PATCH 2/2] KVM: x86: introduce KVM_X86_QUIRK_TSC_HOST

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Date:   Mon, 30 Nov 2020 15:15:11 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org
Cc:     Oliver Upton <oupton@...gle.com>, Ingo Molnar <mingo@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        open list <linux-kernel@...r.kernel.org>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Wanpeng Li <wanpengli@...cent.com>,
        Borislav Petkov <bp@...en8.de>,
        Jim Mattson <jmattson@...gle.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Joerg Roedel <joro@...tes.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [PATCH 2/2] KVM: x86: introduce KVM_X86_QUIRK_TSC_HOST_ACCESS

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@...hat.com>
>>> Signed-off-by: Maxim Levitsky <mlevitsk@...hat.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