On Fri, 2026-07-24 at 14:27 -0700, Sean Christopherson wrote: > On Fri, Jul 03, 2026, David Woodhouse wrote: > > From: David Woodhouse <[email protected]> > > > > 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: [email protected] > > 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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