Re: [PATCH 1/2] x86/irq: fix calculation of max PV dom0 pIRQs

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Nov 21, 2024 at 11:49:44AM +0100, Jan Beulich wrote:
> On 20.11.2024 12:35, Roger Pau Monne wrote:
> > The current calculation of PV dom0 pIRQs uses:
> > 
> > n = min(fls(num_present_cpus()), dom0_max_vcpus());
> > 
> > The usage of fls() is wrong, as num_present_cpus() already returns the 
> > number
> > of present CPUs, not the bitmap mask of CPUs.
> 
> Hmm. Perhaps that use of fls() should have been accompanied by a comment, but
> I think it might have been put there intentionally, to avoid linear growth.
> Which isn't to say that I mind the adjustment, especially now that we don't
> use any clustered modes anymore for I/O interrupts. I'm merely questioning
> the Fixes: tag, and with that whether / how far to backport.

Hm, sorry I've assumed the fls() was a typo.  It seems wrong to cap
dom0 vCPUs with the fls of the present CPUs number.  For consistency,
if the intention was to use fls to limit growth, I would have expected
to also be applied to the dom0 number of vCPUs.  And a comment would
have been nice indeed :).

In any case this is hurting XenServer now: we got reports of pIRQ
exhaustion on some systems.

Thanks, Roger.