From: David Woodhouse The upcoming patches will minimize the chances of updating the master clock data. Unfortunately, this may cause issues in compute_guest_tsc(). Since the KVM clock reference point in master_kernel_ns might actually be *earlier* than the reference point used for the guest TSC (vcpu->last_tsc_nsec), this might lead to a negative delta to be passed to pvclock_scale_delta(). Fix the compute_guest_tsc() function to cope with negative numbers. Signed-off-by: David Woodhouse Link: https://lore.kernel.org/all/20240522001817.619072-11-dwmw2@infradead.org/ [Dongli: copy relevant code from above link and modify changelog] Signed-off-by: Dongli Zhang --- This a partial copy of "[RFC PATCH v3 10/21] KVM: x86: Fix software TSC upscaling in kvm_update_guest_time()", as Sean suggested, "Please do this in a separate patch. There's no need to squeeze it in here, and this change is complex/subtle enough as it is.", and David's authorship is preserved. arch/x86/kvm/x86.c | 28 +++++++++++++++++----------- 1 file changed, 17 insertions(+), 11 deletions(-) diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 63afdb6bb078..5e7418cfd0af 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -2581,10 +2581,19 @@ 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; + s64 delta = kernel_ns - vcpu->arch.this_tsc_nsec; + u64 tsc = vcpu->arch.this_tsc_write; + + /* pvclock_scale_delta cannot cope with negative deltas */ + if (delta >= 0) + tsc += pvclock_scale_delta(delta, + vcpu->arch.virtual_tsc_mult, + vcpu->arch.virtual_tsc_shift); + else + tsc -= pvclock_scale_delta(-delta, + vcpu->arch.virtual_tsc_mult, + vcpu->arch.virtual_tsc_shift); + return tsc; } @@ -2595,7 +2604,7 @@ static inline bool gtod_is_based_on_tsc(int mode) } #endif -static void kvm_track_tsc_matching(struct kvm_vcpu *vcpu, bool new_generation) +static void kvm_track_tsc_matching(struct kvm_vcpu *vcpu) { #ifdef CONFIG_X86_64 struct kvm_arch *ka = &vcpu->kvm->arch; @@ -2612,12 +2621,9 @@ static void kvm_track_tsc_matching(struct kvm_vcpu *vcpu, bool new_generation) /* * Request a masterclock update if the masterclock needs to be toggled - * on/off, or when starting a new generation and the masterclock is - * enabled (compute_guest_tsc() requires the masterclock snapshot to be - * taken _after_ the new generation is created). + * on/off. */ - if ((ka->use_master_clock && new_generation) || - (ka->use_master_clock != use_master_clock)) + if (ka->use_master_clock != use_master_clock) kvm_make_request(KVM_REQ_MASTERCLOCK_UPDATE, vcpu); trace_kvm_track_tsc(vcpu->vcpu_id, ka->nr_vcpus_matched_tsc, @@ -2803,7 +2809,7 @@ static void __kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 offset, u64 tsc, vcpu->arch.this_tsc_nsec = kvm->arch.cur_tsc_nsec; vcpu->arch.this_tsc_write = kvm->arch.cur_tsc_write; - kvm_track_tsc_matching(vcpu, !matched); + kvm_track_tsc_matching(vcpu); } static void kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 *user_value) -- 2.39.3