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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" On Tue, Jul 28, 2026, David Woodhouse wrote: > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index 5c78dd1e4c69..0680332d7d45 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -3435,6 +3435,169 @@ static int kvm_vcpu_ioctl_enable_cap(struct kvm_vcpu *vcpu, > } > } > > +#ifdef CONFIG_X86_64 > +static int kvm_vcpu_ioctl_get_clock_guest(struct kvm_vcpu *v, void __user *argp) > +{ > + struct pvclock_vcpu_time_info hv_clock = {}; > + struct kvm_vcpu_arch *vcpu = &v->arch; > + struct kvm_arch *ka = &v->kvm->arch; > + unsigned int seq; > + > + /* > + * If KVM_REQ_CLOCK_UPDATE is already pending, or if the pvclock > + * has never been generated at all, call kvm_guest_time_update(). > + */ > + if (kvm_check_request(KVM_REQ_CLOCK_UPDATE, v) || !vcpu->hw_tsc_hz) { > + int idx = srcu_read_lock(&v->kvm->srcu); > + int ret = kvm_guest_time_update(v); > + > + srcu_read_unlock(&v->kvm->srcu, idx); guard(srcu)(&vcpu->kvm->srcu); if (kvm_guest_time_update(v)) return -EBUSY; > + if (ret) > + return -EINVAL; This should be -EBUSY, because KVM_REQ_CLOCK_UPDATE is a transient condition. And that's why a capability is needed: if userspace goes with the "probe" method, it could get a temporary failure, and then a naive userspace could stop using the ioctl entirely. > + } > + > + /* > + * Reconstruct the pvclock from the master clock state, matching > + * exactly what kvm_guest_time_update() writes to the guest. > + */ > + do { > + seq = read_seqcount_begin(&ka->pvclock_sc); > + > + if (!ka->use_master_clock) > + return -EINVAL; EINVAL also feels wrong, userspace hasn't done anything wrong. Maybe -ENODATA? Not sure what the right returnis. > + > + hv_clock.tsc_timestamp = kvm_read_l1_tsc(v, ka->master_cycle_now); > + hv_clock.system_time = ka->master_kernel_ns + ka->kvmclock_offset; > + } while (read_seqcount_retry(&ka->pvclock_sc, seq)); > + > + hv_clock.tsc_shift = vcpu->pvclock_tsc_shift; > + hv_clock.tsc_to_system_mul = vcpu->pvclock_tsc_mul; > + hv_clock.flags = ka->all_vcpus_matched_tsc ? PVCLOCK_TSC_STABLE_BIT : 0; > + > + if (copy_to_user(argp, &hv_clock, sizeof(hv_clock))) > + return -EFAULT; > + > + return 0; > +} > + > +/* > + * Reverse the calculation in the hv_clock definition. > + * > + * time_ns = ( (cycles << shift) * mul ) >> 32; > + * (although shift can be negative, so that's bad C) > + * > + * So for a single second, > + * NSEC_PER_SEC = ( ( FREQ_HZ << shift) * mul ) >> 32 > + * NSEC_PER_SEC << 32 = ( FREQ_HZ << shift ) * mul > + * ( NSEC_PER_SEC << 32 ) / mul = FREQ_HZ << shift > + * ( NSEC_PER_SEC << 32 ) / mul ) >> shift = FREQ_HZ > + */ > +static u64 hvclock_to_hz(u32 mul, s8 shift) > +{ > + u64 tm = NSEC_PER_SEC << 32; > + > + /* Maximise precision. Shift right until the top bit is set */ > + tm <<= 2; > + shift += 2; > + > + /* While 'mul' is even, increase the shift *after* the division */ > + while (!(mul & 1)) { > + shift++; > + mul >>= 1; > + } > + > + tm /= mul; > + > + if (shift >= 64) > + return 0; > + if (shift > 0) > + return tm >> shift; > + if (shift <= -64) > + return 0; > + return tm << -shift; > +} > + > +static int kvm_vcpu_ioctl_set_clock_guest(struct kvm_vcpu *v, void __user *argp) > +{ > + struct pvclock_vcpu_time_info user_hv_clock; > + struct kvm *kvm = v->kvm; > + struct kvm_arch *ka = &kvm->arch; > + u64 curr_tsc_hz, user_tsc_hz; > + u64 user_clk_ns; > + u64 guest_tsc; > + int rc = 0; > + > + if (copy_from_user(&user_hv_clock, argp, sizeof(user_hv_clock))) > + return -EFAULT; > + > + if (user_hv_clock.pad0 || user_hv_clock.pad[0] || user_hv_clock.pad[1]) > + return -EINVAL; > + > + if (!user_hv_clock.tsc_to_system_mul) > + return -EINVAL; > + > + if (user_hv_clock.tsc_shift < -31 || user_hv_clock.tsc_shift > 31) > + return -EINVAL; > + > + user_tsc_hz = hvclock_to_hz(user_hv_clock.tsc_to_system_mul, > + user_hv_clock.tsc_shift); > + > + kvm_hv_request_tsc_page_update(kvm); > + > + /* > + * kvm_start_pvclock_update() takes tsc_write_lock and opens > + * the pvclock seqcount; kvm_end_pvclock_update() closes both. > + * All clock state modifications between them are atomic with > + * respect to readers in kvm_guest_time_update(). > + */ > + kvm_start_pvclock_update(kvm); > + pvclock_update_vm_gtod_copy(kvm); > + > + if (!ka->use_master_clock) { > + rc = -EINVAL; > + goto out; > + } > + > + curr_tsc_hz = (u64)get_cpu_tsc_khz() * HZ_PER_KHZ; > + if (unlikely(curr_tsc_hz == 0)) { > + rc = -EINVAL; Same comments here regarding return codes. > + goto out; > + } > + > + if (kvm_caps.has_tsc_control) > + curr_tsc_hz = kvm_scale_tsc(curr_tsc_hz, > + v->arch.l1_tsc_scaling_ratio); > + > + /* > + * Allow for a discrepancy of 1 kHz either way between the TSC > + * frequency used to generate the user's pvclock and the current > + * host's measured frequency, since they may not precisely match. > + */ > + if (user_tsc_hz < curr_tsc_hz - 1000 || > + user_tsc_hz > curr_tsc_hz + 1000) { I don't follow, why is KVM restricting what frequency userspace can set? > + rc = -ERANGE; > + goto out; > + }