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Peter Anvin" , Vitaly Kuznetsov , Juergen Gross , Boris Ostrovsky , David Woodhouse , 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 Subject: [PATCH v6 15/36] KVM: x86: Fix compute_guest_tsc() to handle negative time deltas Date: Fri, 3 Jul 2026 22:17:54 +0100 Message-ID: <20260703212145.343527-16-dwmw2@infradead.org> X-Mailer: git-send-email 2.54.0 In-Reply-To: <20260703212145.343527-1-dwmw2@infradead.org> References: <20260703212145.343527-1-dwmw2@infradead.org> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Sender: David Woodhouse X-SRS-Rewrite: SMTP reverse-path rewritten from by desiato.infradead.org. See http://www.infradead.org/rpr.html X-purgate-ID: tlsNG-d25034/1783113832-92F3BD51-96E90A37/0/0 X-purgate-type: clean X-purgate-size: 1721 X-ZohoMail-DKIM: pass (identity @infradead.org) X-ZM-MESSAGEID: 1783113856907158500 Content-Type: text/plain; charset="utf-8" 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. 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, u= 32 user_tsc_khz) =20 static u64 compute_guest_tsc(struct kvm_vcpu *vcpu, s64 kernel_ns) { - u64 tsc =3D pvclock_scale_delta(kernel_ns-vcpu->arch.this_tsc_nsec, - vcpu->arch.virtual_tsc_mult, - vcpu->arch.virtual_tsc_shift); - tsc +=3D vcpu->arch.this_tsc_write; - return tsc; + s64 delta_ns =3D kernel_ns - vcpu->arch.this_tsc_nsec; + u64 tsc; + + /* Handle negative deltas gracefully (master clock ref may be earlier) */ + if (delta_ns < 0) { + tsc =3D pvclock_scale_delta(-delta_ns, + vcpu->arch.virtual_tsc_mult, + vcpu->arch.virtual_tsc_shift); + return vcpu->arch.this_tsc_write - tsc; + } + + tsc =3D pvclock_scale_delta(delta_ns, + vcpu->arch.virtual_tsc_mult, + vcpu->arch.virtual_tsc_shift); + return vcpu->arch.this_tsc_write + tsc; } =20 #ifdef CONFIG_X86_64 --=20 2.54.0