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

[Luwei Cheng <chengluwei@xxxxxxxxx>]

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.

Jan

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).

Luwei

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