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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

Jan


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