Re: [PATCHv1] xen: increase default number of PIRQs for hardware domains

[David Vrabel <david.vrabel@citrix.com>]

On 05/12/14 09:44, Jan Beulich wrote:
>>>> On 03.12.14 at 17:04, <david.vrabel@citrix.com> wrote:
>> The default limit for the number of PIRQs for hardware domains (dom0)
>> is not sufficient for some (x86) systems.
>>
>> Since the pirq structures are individually and dynamically allocated,
>> the limit for hardware domains may be increased to the number of
>> possible IRQs.
> 
> I nevertheless disagree to moving the bound up to the Xen internal
> limit unconditionally: What use does it have to allow hwdom to use
> thousands of MSIs? If a system got that many, the main purpose of
> running Xen on it I would expect to be to hand various of the
> respective devices to guests. Hence no need for hwdom to have
> that many by default, even if this doesn't result in any extra
> resource consumption.
> 
> That said, I can see the current default of 256 being too low though.
> Quite likely in the absence of a user specified value the default
> ought to be derived from nr_irqs - nr_static_irqs rather than being
> any fixed number. Considering the default used for nr_irqs, I'd think
> along the lines of sqrt(num_present_cpus()) * NR_DYNAMIC_VECTORS
> or dom0->max_vcpus * NR_DYNAMIC_VECTORS (or the minimum of
> the two) for x86.

The reason for a non obvious default like this would need a comment and
I can't write one because I can't see a reason for it.  Perhaps if you
write a suitable comment for your preferred default I can respin this patch?

David