Re: [PATCH v6 8/8] xen: allow up to 16383 cpus

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Mon, 29 Apr 2024, Julien Grall wrote:
> Hi Juergen,
> 
> On 29/04/2024 12:28, Jürgen Groß wrote:
> > On 29.04.24 13:04, Julien Grall wrote:
> > > Hi Juergen,
> > > 
> > > Sorry for the late reply.
> > > 
> > > On 29/04/2024 11:33, Juergen Gross wrote:
> > > > On 08.04.24 09:10, Jan Beulich wrote:
> > > > > On 27.03.2024 16:22, Juergen Gross wrote:
> > > > > > With lock handling now allowing up to 16384 cpus (spinlocks can
> > > > > > handle
> > > > > > 65535 cpus, rwlocks can handle 16384 cpus), raise the allowed limit
> > > > > > for
> > > > > > the number of cpus to be configured to 16383.
> > > > > > 
> > > > > > The new limit is imposed by IOMMU_CMD_BUFFER_MAX_ENTRIES and
> > > > > > QINVAL_MAX_ENTRY_NR required to be larger than 2 * CONFIG_NR_CPUS.
> > > > > > 
> > > > > > Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> > > > > 
> > > > > Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
> > > > > 
> > > > > I'd prefer this to also gain an Arm ack, though.
> > > > 
> > > > Any comment from Arm side?
> > > 
> > > Can you clarify what the new limits mean in term of (security) support?
> > > Are we now claiming that Xen will work perfectly fine on platforms with up
> > > to 16383?
> > > 
> > > If so, I can't comment for x86, but for Arm, I am doubtful that it would
> > > work without any (at least performance) issues. AFAIK, this is also an
> > > untested configuration. In fact I would be surprised if Xen on Arm was
> > > tested with more than a couple of hundreds cores (AFAICT the Ampere CPUs
> > > has 192 CPUs).
> > 
> > I think we should add a security support limit for the number of physical
> > cpus similar to the memory support limit we already have in place.
> > 
> > For x86 I'd suggest 4096 cpus for security support (basically the limit we
> > have with this patch), but I'm open for other suggestions, too.
> > 
> > I have no idea about any sensible limits for Arm32/Arm64.
> 
> I am not entirely. Bertrand, Michal, Stefano, should we use 192 (the number of
> CPUs from Ampere)?

I am OK with that. If we want to be a bit more future proof we could say
256 or 512. I leave this to you, as all the boards I have have a much
smaller CPU count.