Re: [PATCH] KVM: x86: Don't unnecessarily force masterclock update on vCPU hotplug

[Sean Christopherson]

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

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https://github.com/kvm-x86/linux/tree/next