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

[David Woodhouse]

On Fri, 2022-02-25 at 16:21 +0000, Sean Christopherson wrote:
> > 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.


Gotcha.

So even when we put my patch in front, to snapshot a value into
kvm->arch.default_tsc_khz, that's happening later at VM creation time
so should also be snapshotting the *good* value.

And at least if it snapshots the bad value, all the vCPUs will be
*consistent*.

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