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    KVM: x86: Don't snapshot "max" TSC if host TSC is constant · 741e511b
    Sean Christopherson 提交于
    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.
    
    [ dwmw2: Rebase on top of KVM-wide default_tsc_khz to ensure that all
             vCPUs get the same frequency even if we hit the race. ]
    
    Cc: Suleiman Souhlal <suleiman@google.com>
    Cc: Anton Romanov <romanton@google.com>
    Signed-off-by: NSean Christopherson <seanjc@google.com>
    Signed-off-by: NDavid Woodhouse <dwmw@amazon.co.uk>
    Message-Id: <20220225145304.36166-3-dwmw2@infradead.org>
    Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
    741e511b
x86.c 340.3 KB