Messages in this thread | | | Date | Fri, 29 Sep 2023 14:26:05 +0100 | Subject | Re: [PATCH v2] KVM: x86: Use fast path for Xen timer delivery | From | "Durrant, Paul" <> |
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On 29/09/2023 12:36, David Woodhouse wrote: > From: David Woodhouse <dwmw@amazon.co.uk> > > 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@redhat.com/ > > Signed-off-by: David Woodhouse <dwmw@amazon.co.uk> > --- > > • 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@xen.org>
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