linux-kernel - Re: [PATCH] KVM: x86: Don't snapshot "max" TSC if host TSC is constant

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Date:   Fri, 25 Feb 2022 16:21:24 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Suleiman Souhlal <suleiman@...gle.com>,
        Anton Romanov <romanton@...gle.com>
Subject: Re: [PATCH] KVM: x86: Don't snapshot "max" TSC if host TSC is
 constant

On Fri, Feb 25, 2022, David Woodhouse wrote:
> On Fri, 2022-02-25 at 13:10 +0100, Paolo Bonzini wrote:
> > 
> > Queued, but I'd rather have a subject that calls out that max_tsc_khz 
> > needs a replacement at vCPU creation time.  In fact, the real change 
> > (and bug, and fix) is in kvm_arch_vcpu_create(), while the subject 
> > mentions only the change in kvm_timer_init().
> 
> In
> https://lore.kernel.org/kvm/e7be32b06676c7ebf415d9deea5faf50aa8c0785.camel@infradead.org/T/
> last night I was coming round to the idea that we might want a KVM-wide 
> default frequency which is settable from userspace and is used instead
> of max_tsc_khz anyway.
> 
> I also have questions about the use case for the above patch.... if
> this is a clean boot and you're just starting to host guests, surely we
> can wait for the time it takes for the TSC synchronization to complete?

KVM is built into the kernel in their case, the vmx_init() => kvm_init() gets
automatically called during boot.  The VMs aren't started until well after
synchronization has completed, but KVM has already snapshotted the "bad" value.