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

[Sean Christopherson <seanjc@xxxxxxxxxx>]

On Sat, Jul 25, 2026, David Woodhouse wrote:
> 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.

Heh, in that case, throw a blurb in the changelog stating as much.  Knowing that
a bug is likely unreachable in practice is helpful, e.g. in the super unlikely
case that this change breaks someone.

> 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...