Re: [PATCH v2] KVM: x86: Use fast path for Xen timer delivery

[Durrant, Paul]

On 29/09/2023 12:36, David Woodhouse wrote:
> From: David Woodhouse <dwmw@xxxxxxxxxxxx>
>
> Most of the time there's no need to kick the vCPU and deliver the timer
> event through kvm_xen_inject_timer_irqs(). Use kvm_xen_set_evtchn_fast()
> directly from the timer callback, and only fall back to the slow path
> when it's necessary to do so.
>
> This gives a significant improvement in timer latency testing (using
> nanosleep() for various periods and then measuring the actual time
> elapsed).
>
> However, there was a reason¹ the fast path was dropped when this support
> was first added. The current code holds vcpu->mutex for all operations on
> the kvm->arch.timer_expires field, and the fast path introduces potential
> race conditions. So... ensure the hrtimer is *cancelled* before making
> changes in kvm_xen_start_timer(), and also when reading the values out
> for KVM_XEN_VCPU_ATTR_TYPE_TIMER.
>
> Add some sanity checks to ensure the truth of the claim that all the
> other code paths are run with the vcpu loaded. And use hrtimer_cancel()
> directly from kvm_xen_destroy_vcpu() to avoid a false positive from the
> check in kvm_xen_stop_timer().
>
> ¹ https://lore.kernel.org/kvm/846caa99-2e42-4443-1070-84e49d2f11d2@xxxxxxxxxx/
>
> Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
> ---
>
>   • v2: Remember, and deal with, those races.
>
>   arch/x86/kvm/xen.c | 64 +++++++++++++++++++++++++++++++++++++++++-----
>   1 file changed, 58 insertions(+), 6 deletions(-)

Reviewed-by: Paul Durrant <paul@xxxxxxx>