Re: [Xen-devel] [PROPOSAL] Event channel for SMP-VMs: per-vCPU or per-OS?

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 29.10.13 at 10:49, Luwei Cheng <chengluwei@xxxxxxxxx> wrote:
> On Tue, Oct 29, 2013 at 5:34 PM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
>> >>> On 29.10.13 at 10:02, Luwei Cheng <chengluwei@xxxxxxxxx> wrote:
>> > On Tue, Oct 29, 2013 at 4:19 PM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
>> >> >>> On 29.10.13 at 03:56, Luwei Cheng <chengluwei@xxxxxxxxx> wrote:
>> >> > Hmm.. though all vCPUs can serve the events, the hypervisor delivers the
>> >> > event to only "one" vCPU at at time, so only that vCPU can see this 
>> >> > event.
>> >> > Analytically no race condition will be introduced.
>> >>
>> >> No - an event is globally pending (at least in the old model, the
>> >> situation is better with the new FIFO model), i.e. if more than
>> >> one of the guest's vCPU-s allowed to service it would be looking
>> >> at it simultaneously, they'd still need to arbitrate which one
>> >> ought to handle it.
>> >>
>> >> So your proposed extension might need to be limited to the
>> >> FIFO model.
>> >
>> > Thanks for your reply. Yes, you are right. My prior description was
>> > incorrect.
>> > When there are more than one vCPUs picking the event, even without
>> > arbitrary, will it cause "correctness" problem? After the event is
>> served by
>> > the first entered vCPU, and the rest vCPUs just have noting to do in the
>> > event handler (no much harm).
>>
>> That really depends on the handler. Plus it might be a performance
>> and/or latency issue to run handlers that don't need to be run.
> 
> I think the situation is much like IO-APIC routing in physical SMP
> systems:

Indeed, yet you draw the wrong conclusion.

> in logical destination mode, all processors can serve I/O interrupts.

But only one gets delivered any individual instance - there is
arbitration being done in hardware.

Jan


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