On Mon, 2016-04-04 at 16:46 -0400, Luiz Capitulino wrote:
> When a vCPU runs on a nohz_full core, the hrtimer used by
> the lapic emulation code can be migrated to another core.
> When this happens, it's possible to observe milisecond
> latency when delivering timer IRQs to KVM guests.
> 
> The huge latency is mainly due to the fact that
> apic_timer_fn() expects to run during a kvm exit. It
> sets KVM_REQ_PENDING_TIMER and let it be handled on kvm
> entry. However, if the timer fires on a different core,
> we have to wait until the next kvm exit for the guest
> to see KVM_REQ_PENDING_TIMER set.
> 
> This problem became visible after commit 9642d18ee. This
> commit changed the timer migration code to always attempt
> to migrate timers away from nohz_full cores. While it's
> discussable if this is correct/desirable (I don't think
> it is), it's clear that the lapic emulation code has
> a requirement on firing the hrtimer in the same core
> where it was started. This is achieved by making the
> hrtimer pinned.

Given that delivering a timer to a guest seems to
involve trapping from the guest to the host, anyway,
I don't see a downside to your patch.

If that is ever changed (eg. allowing delivery of
a timer interrupt to a VCPU without trapping to the
host), we may want to revisit this.

Until then...

Acked-by: Rik van Riel <riel@redhat.com>

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