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

["Orzel, Michal" <michal.orzel@xxxxxxx>]

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).

> +when it is the hardware domain.  In this case, this value is also limited
> +by the redistributor MMIO regions that can be exposed for that layout.
> +Xen can only create Dom0 VCPUs whose virtual redistributor frame is
> +covered by those regions.  If the requested number of VCPUs exceeds that
> +limit, secondary VCPU creation stops at the first VCPU whose virtual
> +redistributor frame is not covered.
> +
>  Or:
>  
>  > `= <min>-<max>` where `<min>` and `<max>` are integers.

~Michal