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

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Message-ID: <609de7ff-92e2-f96e-e6f5-127251f6e16d@redhat.com>
Date:   Fri, 25 Feb 2022 13:10:12 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     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 2/25/22 02:39, Sean Christopherson wrote:
> Don't snapshot tsc_khz into max_tsc_khz during KVM initialization if the
> host TSC is constant, in which case the actual TSC frequency will never
> change and thus capturing the "max" TSC during initialization is
> unnecessary, KVM can simply use tsc_khz during VM creation.
> 
> On CPUs with constant TSC, but not a hardware-specified TSC frequency,
> snapshotting max_tsc_khz and using that to set a VM's default TSC
> frequency can lead to KVM thinking it needs to manually scale the guest's
> TSC if refining the TSC completes after KVM snapshots tsc_khz.  The
> actual frequency never changes, only the kernel's calculation of what
> that frequency is changes.  On systems without hardware TSC scaling, this
> either puts KVM into "always catchup" mode (extremely inefficient), or
> prevents creating VMs altogether.
> 
> Ideally, KVM would not be able to race with TSC refinement, or would have
> a hook into tsc_refine_calibration_work() to get an alert when refinement
> is complete.  Avoiding the race altogether isn't practical as refinement
> takes a relative eternity; it's deliberately put on a work queue outside
> of the normal boot sequence to avoid unnecessarily delaying boot.
> 
> Adding a hook is doable, but somewhat gross due to KVM's ability to be
> built as a module.  And if the TSC is constant, which is likely the case
> for every VMX/SVM-capable CPU produced in the last decade, the race can
> be hit if and only if userspace is able to create a VM before TSC
> refinement completes; refinement is slow, but not that slow.
> 
> For now, punt on a proper fix, as not taking a snapshot can help some
> uses cases and not taking a snapshot is arguably correct irrespective of
> the race with refinement.
> 
> Cc: Suleiman Souhlal <suleiman@...gle.com>
> Cc: Anton Romanov <romanton@...gle.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

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().

What do you think of "KVM: x86: Use current rather than max TSC 
frequency if it is constant"?

Pao