Xen (and others Hypervisors) how do they handle IPIs?

[Roger Pau Monné]

On 25/09/13 16:56, Julian Elischer wrote:
> If CPUs are mapped around, how are IPIs handled? I assume they must be
> emulated?
> 
> I've noticed that under Xen (on both Amazon EC2 and a Redhat server)
> whenever you schedule a thread it always sits on the run queue for 20
> uSecs before it starts running. It looks to me like it's the IPI taking
> a long time to be emulated.

This has been improved on the FreeBSD Xen PVHVM port by using PV IPIs
instead of the emulated ones, see r255331. It should be faster than the
previous emulated implementation.