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 12:39:06PM +0100, Jan Beulich wrote:
> On 21.11.2024 12:04, Roger Pau Monné wrote:
> > 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.
> 
> FTR: My vague recollection (it has been nearly 10 years) is that I first had
> it there, too. Until I realized that it hardly ever would have any effect,
> because of the min(). And for Dom0-s with extremely few vCPU-s it seemed
> reasonable to not apply that cap there.

I don't have a strong opinion regarding the fixes tag, but would like
to get this sorted as soon as possible.  If you prefer just drop the
fixes tag.  I think this wants backporting to all supported releases,
because it's an issue XenServer has hit on real servers when dom0 is
sized to use 16 vCPUs at most.

Thanks, Roger.