Re: [PATCH 2/2] docs: Document Arm GICv3 limit for dom0_max_vcpus

[Mykola Kvach <xakep.amatop@xxxxxxxxx>]

Hi Michal,

Thank you for the review.

On Mon, May 25, 2026 at 12:55 PM Orzel, Michal <michal.orzel@xxxxxxx> wrote:
>
>
>
> On 22-May-26 08:18, Mykola Kvach wrote:
> > From: Mykola Kvach <mykola_kvach@xxxxxxxx>
> >
> > On Arm GICv3 systems, Dom0 may use the host GIC layout, for example
> > when it is the hardware domain. In that case the effective Dom0 vCPU
> > count is not only bounded by the requested dom0_max_vcpus value and
> > Xen's generic vCPU limits. It is also bounded by the redistributor MMIO
> > regions that can be exposed for the host layout.
> >
> > Document that Xen can only create Dom0 vCPUs whose virtual redistributor
> > frame is covered by those regions, and that secondary vCPU creation stops
> > at the first vCPU whose virtual redistributor frame is not covered.
> >
> > Signed-off-by: Mykola Kvach <mykola_kvach@xxxxxxxx>
> > ---
> >  docs/misc/xen-command-line.pandoc | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> >
> > diff --git a/docs/misc/xen-command-line.pandoc 
> > b/docs/misc/xen-command-line.pandoc
> > index d46d5b2152..0a5589b24c 100644
> > --- a/docs/misc/xen-command-line.pandoc
> > +++ b/docs/misc/xen-command-line.pandoc
> > @@ -981,6 +981,14 @@ The number of VCPUs to give to dom0.  This number of 
> > VCPUs can be more
> >  than the number of PCPUs on the host.  The default is the number of
> >  PCPUs.
> >
> > +On Arm systems using GICv3, Dom0 may use the host GIC layout, for example
> It's not dom0 specific. It can by any domain using host memory layout meaning
> dom0, hwdom, domU direct mapped, in which case I'm not sure if such comment
> should belong here given that this behavior is not dom0_max_vcpus specific 
> (same
> could be triggered on your setup with dom0less domU, direct-mapped and 17 
> vCPUs).

You are right that the redistributor coverage limit is not specific to
dom0_max_vcpus. It applies to Arm GICv3 domains using the host layout,
such as the hardware domain or a direct-mapped domain.

My reason for touching dom0_max_vcpus was that this option is where users
are told how many Dom0 VCPUs Xen will try to create. However, the
GICv3-specific details would fit better in a more generic Arm
device-tree/domain description section, around the cpus/direct-map
properties.

For dom0_max_vcpus, I can keep only a short generic note, for example:

The resulting number of Dom0 VCPUs is still subject to Xen's generic
and architecture-specific domain limits.

~Mykola