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

[Julien Grall <julien@xxxxxxx>]

Hi,

On 08/04/2022 16:26, Roger Pau Monné 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?
Looking at the x86 implementation, I think we can re-use everything but 
the pv_shim and NUMA bits.
> We don't need to store this information anywhere I think. Since we
> first install plain Debian and then install Xen we can always fetch
> the number of physical CPUs when running plain Linux and use that to
> calculate the amount to give to dom0?
You will need to check how that works with U-boot. I can't remember 
whether the script is loaded via tftp or stored on the local disk.
Cheers,

--
Julien Grall