Re: [PATCH v6 15/36] KVM: x86: Fix compute_guest_tsc() to handle negative time deltas

[David Woodhouse <dwmw2@xxxxxxxxxxxxx>]

On Fri, 2026-07-24 at 14:27 -0700, Sean Christopherson wrote:
> On Fri, Jul 03, 2026, David Woodhouse wrote:
> > From: David Woodhouse <dwmw@xxxxxxxxxxxx>
> > 
> > The compute_guest_tsc() function computes the guest TSC at a given
> > kernel_ns timestamp. When the master clock reference point
> > (master_kernel_ns) is earlier than vcpu->arch.this_tsc_nsec, the delta
> > is negative. Since pvclock_scale_delta() takes a u64, the negative
> > value wraps to a huge positive number, producing a wildly wrong result.
> > 
> > Handle negative deltas explicitly by negating the delta, scaling it,
> > and subtracting from this_tsc_write.
> 
> Does this need 
> 
>   Cc: stable@xxxxxxxxxxxxxxx
> 
> or is this a "technically a bug fix, but can't happen in practice" sort of 
> thing?

Honestly, there's so much hosed in kvmclock that we could debate a
cc:stable for half of this series. I'm not doing *any* of this to
actually add new features. For this one I'm not sure it's reachable in
practice; I think I did it mostly to shut Sashiko up.

I think even in the KVM_SET_CLOCK_GUEST it can't trigger, although I
have fantasies about changing the way KVM_SET_CLOCK_GUEST works to fix
that final ±1ns imprecision (which requires separate work on the
timekeeping core), that *might* trigger the negative delta here...

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