Re: [xen-4.12-testing test] 169199: regressions - FAIL

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Fri, 8 Apr 2022, Andrew Cooper wrote:
> On 08/04/2022 16:26, Roger Pau Monne wrote:
> > On Fri, Apr 08, 2022 at 12:24:27PM +0100, Julien Grall wrote:
> >> Hi Roger,
> >>
> >> On 08/04/2022 12:16, Roger Pau Monné wrote:
> >>> On Fri, Apr 08, 2022 at 12:08:02PM +0100, Julien Grall wrote:
> >>>> Hi,
> >>>>
> >>>> On 08/04/2022 12:01, Roger Pau Monné wrote:
> >>>>>>> I could add a suitable dom0_max_vcpus parameter to osstest.  XenServer
> >>>>>>> uses 16 for example.
> >>>>>> I'm afraid a fixed number won't do, the more that iirc there are
> >>>>>> systems with just a few cores in the pool (and you don't want to
> >>>>>> over-commit by default).
> >>>>> But this won't over commit, it would just assign dom0 16 vCPUs at
> >>>>> most, if the system has less than 16 vCPUs that's what would be
> >>>>> assigned to dom0.
> >>>> AFAICT, this is not the case on Arm. If you ask 16 vCPUs, then you will 
> >>>> get
> >>>> that number even if there are 8 pCPUs.
> >>>>
> >>>> In fact, the documentation of dom0_max_vcpus suggests that the numbers of
> >>>> vCPUs can be more than the number of pCPUs.
> >>> It was my understanding that you could only achieve that by using the
> >>> min-max nomenclature, so in order to force 16 vCPUs always you would
> >>> have to use:
> >>>
> >>> dom0_max_vcpus=16-16
> >>>
> >>> Otherwise the usage of '_max_' in the option name is pointless, and it
> >>> should instead be dom0_vcpus.
> >>>
> >>> Anyway, I could use:
> >>>
> >>> dom0_max_vcpus=1-16
> >>>
> >>> Which is unambiguous and should get us 1 vCPU at least, or 16vCPUs at
> >>> most.
> >> Unfortunately, Arm doesn't support the min-max nomenclature.
> > Hm, can we update the command line document then?
> >
> > There's no mention that the min-max nomenclature is only available to
> > x86. I assume it's not possible to share the logic here so that both
> > Arm and x86 parse the option in the same way?
> 
> TBH, this especially wants moving to common code.  It's atrocious UX to
> have per-arch variations on the syntax for "how many vcpus".

In my view, it would be OK to share the code, but I would also want to
retain the current behavior when e.g. dom0_max_vcpus=2 is specified.
Otherwise we break existing ARM tooling. (It is actually used by Yocto
and others.)