Re: [RFC PATCH v3 15/21] KVM: x86: Allow KVM master clock mode when TSCs are offset from each other

[Paul Durrant]

On 22/05/2024 01:17, David Woodhouse wrote:
> From: David Woodhouse <dwmw@xxxxxxxxxxxx>
>
> There is no reason why the KVM clock cannot be in masterclock mode when
> the TSCs are not in sync, as long as they are at the same *frequency*.
>
> Running at a different frequency would lead to a systemic skew between
> the clock(s) as observed by different vCPUs due to arithmetic precision
> in the scaling. So that should indeed force the clock to be based on the
> host's CLOCK_MONOTONIC_RAW instead of being in masterclock mode where it
> is defined by the (or 'a') guest TSC.
>
> But when the vCPUs merely have a different TSC *offset*, that's not a
> problem. The offset is applied to that vCPU's kvmclock->tsc_timestamp
> field, and it all comes out in the wash.
>
> So, remove ka->nr_vcpus_matched_tsc and replace it with a new field
> ka->all_vcpus_matched_tsc which is not only changed to a boolean, but
> also now tracks that the *frequency* matches, not the precise offset.
>
> Using a *count* was always racy because a new vCPU could be being
> created *while* kvm_track_tsc_matching() was running and comparing with
> kvm->online_vcpus. That variable is only atomic with respect to itself.
> In particular, kvm_arch_vcpu_create() runs before kvm->online_vcpus is
> incremented for the new vCPU, and kvm_arch_vcpu_postcreate() runs later.
>
> Repurpose kvm_track_tsc_matching() to be called from kvm_set_tsc_khz(),
> and kill the cur_tsc_generation/last_tsc_generation fields which tracked
> the precise TSC matching.
>
> Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
> ---
>   arch/x86/include/asm/kvm_host.h |   6 +-
>   arch/x86/kvm/x86.c              | 130 +++++++++++++++++---------------
>   2 files changed, 71 insertions(+), 65 deletions(-)

Reviewed-by: Paul Durrant <paul@xxxxxxx>