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

[Luwei Cheng <chengluwei@xxxxxxxxx>]

On Tue, Oct 29, 2013 at 10:42 PM, Jan Beulich <JBeulich@xxxxxxxx> wrote:

>>> On 29.10.13 at 15:28, Luwei Cheng <chengluwei@xxxxxxxxx> wrote:
> On Tue, Oct 29, 2013 at 7:22 PM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
>> >>> On 29.10.13 at 11:52, George Dunlap <george.dunlap@xxxxxxxxxxxxx>
>> wrote:
>> > Xen should be able to arbitrate which one gets the actual event
>> > delivery, right?  So the only risk would be that another vcpu would
>> > notice the pending interrupt and handle it itself.
>>
>> As said before - for the FIFO model Xen's arbitration would be
>> sufficient (as long as affinity changes get carried out with
>> sufficient care), but for the legacy model several vCPU-s might
>> end up trying to service the event (since the pending bitmap is
>> per-domain, not per-vCPU)..
>>

>> As long as the event can be served quickly, and meanwhile there is no
> correctness
> problem (hopefully), do we really care which vCPU serves it..:d ?

No, we don't care. But we do care that it is exactly one that
does.

Jan

Since the I/O event is marked as pending for "the whole guest OS", not

pending for a specific vCPU. To tickle a vCPU does not necessarily mean 

that the event is exactly for that vCPU, but can mean that we give the 

tickled vCPU a chance to serve it. But we do not refuse other vCPUs to 

get in early.

Not sure whether my argument is sensible or not..:)

Thanks,

Luwei

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