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

[Mykola Kvach <xakep.amatop@xxxxxxxxx>]

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
+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.
-- 
2.43.0