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

[Julian Elischer]

On 9/25/13 11:04 PM, Roger Pau Monné wrote:
> 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.
>
I missed that.. thanks!
Do you (or anyone else) know if this can be used on Amazon EC2?
And do you need a specific version/configuration of Xen to be able to 
use it?