Improve the 'highmem' option docs to note that by default we assume
that a 32-bit kernel on an LPAE-capable CPU has LPAE enabled, and
what the consequences are.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
---
 docs/system/arm/virt.rst | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/docs/system/arm/virt.rst b/docs/system/arm/virt.rst
index 7c4c80180c6..c245c52b7ac 100644
--- a/docs/system/arm/virt.rst
+++ b/docs/system/arm/virt.rst
@@ -96,7 +96,13 @@ mte
 highmem
   Set ``on``/``off`` to enable/disable placing devices and RAM in physical
   address space above 32 bits. The default is ``on`` for machine types
-  later than ``virt-2.12``.
+  later than ``virt-2.12`` when the CPU supports an address space
+  bigger than 32 bits (i.e. 64-bit CPUs, and 32-bit CPUs with the
+  Large Physical Address Extension (LPAE) feature). If you want to
+  boot a 32-bit kernel which does not have ``CONFIG_LPAE`` enabled on
+  a CPU type which implements LPAE, you will need to manually set
+  this to ``off``; otherwise some devices, such as the PCI controller,
+  will not be accessible.
 
 compact-highmem
   Set ``on``/``off`` to enable/disable the compact layout for high memory regions.
-- 
2.34.1