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

[Jan Beulich <jbeulich@suse.com>]

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.

Jan