Re: [Xen-devel] [PATCH 2/2] x86/x2apic: properly implement cluster mode

[Keir Fraser <keir@xxxxxxx>]

On 08/11/2012 15:38, "Jan Beulich" <JBeulich@xxxxxxxx> wrote:

>>>> On 08.11.12 at 16:17, Keir Fraser <keir@xxxxxxx> wrote:
>> On 08/11/2012 15:03, "Jan Beulich" <JBeulich@xxxxxxxx> wrote:
>> 
>>> So far, cluster mode was just an alternative implementation of
>>> physical mode: Allowing only single CPU interrupt targets, and sending
>>> IPIs to each target CPU separately. Take advantage of what cluster
>>> mode really can do in that regard.
>> 
>> What does it allow? Multicast within certain constraints?
> 
> For IPIs, yes. For interrupts, this gets things merely in line with
> cluster mode APIC, specifying more than one destination with
> lowest priority delivery. Without actively using the TPR, that
> should only affect nested interrupts (which previously would
> have got deferred until the outer one finished if their vector
> was a lower priority one than the one in service, whereas now
> they could get serviced by another CPU within the cluster).
>
> I also should have pointed out that this has a potential drawback
> - it might increase the pressure on vector allocation, since
> they're now being shared more heavily (within clusters). But if
> someone runs into problems with that, they ought to simply
> switch to physical mode (which previously really had no practical
> use at all).
> 
>> I know it's not
>> part of our coding style, but some comments would be nice. ;)
> 
> Hard to tell what specifically you would want to see commented,
> given that this really just extends the originally rather simplistic
> implementation.

Okay, yes, it doesn't look so complicated re-reading it again. I'm fine with
it as it is.

 -- Keir

> Jan
> 



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