On Wed, 18 Oct 2023 12:56:38 -0700, Sean Christopherson wrote: > Don't force a masterclock update when a vCPU synchronizes to the current > TSC generation, e.g. when userspace hotplugs a pre-created vCPU into the > VM. Unnecessarily updating the masterclock is undesirable as it can cause > kvmclock's time to jump, which is particularly painful on systems with a > stable TSC as kvmclock _should_ be fully reliable on such systems. > > The unexpected time jumps are due to differences in the TSC=>nanoseconds > conversion algorithms between kvmclock and the host's CLOCK_MONOTONIC_RAW > (the pvclock algorithm is inherently lossy). When updating the > masterclock, KVM refreshes the "base", i.e. moves the elapsed time since > the last update from the kvmclock/pvclock algorithm to the > CLOCK_MONOTONIC_RAW algorithm. Synchronizing kvmclock with > CLOCK_MONOTONIC_RAW is the lesser of evils when the TSC is unstable, but > adds no real value when the TSC is stable. > > [...] Applied to kvm-x86 misc, thanks! [1/1] KVM: x86: Don't unnecessarily force masterclock update on vCPU hotplug https://github.com/kvm-x86/linux/commit/c52ffadc65e2 -- https://github.com/kvm-x86/linux/tree/next