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Fri, 24 Jul 2026 14:27:08 -0700 (PDT) Date: Fri, 24 Jul 2026 14:27:07 -0700 In-Reply-To: <20260703212145.343527-16-dwmw2@infradead.org> Mime-Version: 1.0 References: <20260703212145.343527-1-dwmw2@infradead.org> <20260703212145.343527-16-dwmw2@infradead.org> Message-ID: Subject: Re: [PATCH v6 15/36] KVM: x86: Fix compute_guest_tsc() to handle negative time deltas From: Sean Christopherson To: David Woodhouse Cc: Paolo Bonzini , Jonathan Corbet , Shuah Khan , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , Vitaly Kuznetsov , Juergen Gross , Boris Ostrovsky , Paul Durrant , Jonathan Cameron , Sascha Bischoff , Marc Zyngier , Joey Gouly , Jack Allister , Dongli Zhang , joe.jin@oracle.com, kvm@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, xen-devel@lists.xenproject.org, linux-kselftest@vger.kernel.org Content-Type: text/plain; charset="us-ascii" X-purgate-ID: tlsNG-c1860d/1784928430-D594A87B-1B8F9513/0/0 X-purgate-type: clean X-purgate-size: 2392 On Fri, Jul 03, 2026, David Woodhouse wrote: > From: David Woodhouse > > 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@vger.kernel.org or is this a "technically a bug fix, but can't happen in practice" sort of thing? > > Signed-off-by: David Woodhouse > --- > arch/x86/kvm/x86.c | 20 +++++++++++++++----- > 1 file changed, 15 insertions(+), 5 deletions(-) > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index 376b8dc2ade9..55fb19fb7a88 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -2588,11 +2588,21 @@ static int kvm_set_tsc_khz(struct kvm_vcpu *vcpu, u32 user_tsc_khz) > > static u64 compute_guest_tsc(struct kvm_vcpu *vcpu, s64 kernel_ns) > { > - u64 tsc = pvclock_scale_delta(kernel_ns-vcpu->arch.this_tsc_nsec, > - vcpu->arch.virtual_tsc_mult, > - vcpu->arch.virtual_tsc_shift); > - tsc += vcpu->arch.this_tsc_write; > - return tsc; > + s64 delta_ns = kernel_ns - vcpu->arch.this_tsc_nsec; > + u64 tsc; > + > + /* Handle negative deltas gracefully (master clock ref may be earlier) */ > + if (delta_ns < 0) { > + tsc = pvclock_scale_delta(-delta_ns, > + vcpu->arch.virtual_tsc_mult, > + vcpu->arch.virtual_tsc_shift); > + return vcpu->arch.this_tsc_write - tsc; > + } > + > + tsc = pvclock_scale_delta(delta_ns, > + vcpu->arch.virtual_tsc_mult, > + vcpu->arch.virtual_tsc_shift); > + return vcpu->arch.this_tsc_write + tsc; To cut down on the duplicate code, and IMO to make it easier to identify the differences, how about this? s64 delta_ns = kernel_ns - vcpu->arch.this_tsc_nsec; u64 tsc; /* Handle negative deltas gracefully (master clock ref may be earlier) */ tsc = pvclock_scale_delta(abs(-delta_ns), vcpu->arch.virtual_tsc_mult, vcpu->arch.virtual_tsc_shift); return vcpu->arch.this_tsc_write + (delta_ns >= 0 ? tsc : -tsc);