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

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Date:   Mon, 30 Nov 2020 17:33:15 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Paolo Bonzini <pbonzini@...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 Mon, 2020-11-30 at 15:15 +0100, Paolo Bonzini wrote:
> 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 :)).

I'll prepare a test then for it!

Best regards,
	Maxim Levitsky
> 
> Paolo
>