Re: [RFC PATCH v3 13/21] KVM: x86: Improve synchronization in kvm_synchronize_tsc()

[Paul Durrant]

On 22/05/2024 01:17, David Woodhouse wrote:
> From: David Woodhouse <dwmw@xxxxxxxxxxxx>
>
> When synchronizing to an existing TSC (either by explicitly writing zero,
> or the legacy hack where the TSC is written within one second's worth of
> the previously written TSC), the last_tsc_write and last_tsc_nsec values
> were being misrecorded by __kvm_synchronize_tsc(). The *unsynchronized*
> value of the TSC (perhaps even zero) was bring recorded, along with the
> current time at which kvm_synchronize_tsc() was called. This could cause
> *subsequent* writes to fail to synchronize correctly.
>
> Fix that by resetting {data, ns} to the previous values before passing
> them to __kvm_synchronize_tsc() when synchronization is detected. Except
> in the case where the TSC is unstable and *has* to be synthesised from
> the host clock, in which case attempt to create a nsec/tsc pair which is
> on the correct line.
>
> Furthermore, there were *three* different TSC reads used for calculating
> the "current" time, all slightly different from each other. Fix that by
> using kvm_get_time_and_clockread() where possible and using the same
> host_tsc value in all cases.
>
> Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
> ---
>   arch/x86/kvm/x86.c | 32 ++++++++++++++++++++++++++++----
>   1 file changed, 28 insertions(+), 4 deletions(-)

Reviewed-by: Paul Durrant <paul@xxxxxxx>