[PATCH for-4.19 v2] docs/arm: Document where Xen should be loaded in memory

[Julien Grall <julien@xxxxxxx>]

From: Julien Grall <jgrall@xxxxxxxxxx>

In commit 9d267c049d92 ("xen/arm64: Rework the memory layout"),
we decided to require Xen to be loaded below 2 TiB to simplify
the logic to enable the MMU. The limit was decided based on
how known platform boot plus some slack.

We had a recent report that this is not sufficient on the AVA
platform with a old firmware [1]. But the restriction is not
going to change in Xen 4.18. So document the limit clearly
in docs/misc/arm/booting.txt.

[1] https://lore.kernel.org/20231013122658.1270506-3-leo.yan@xxxxxxxxxx

Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>
Reviewed-by: Michal Orzel <michal.orzel@xxxxxxx>

---

    Changes in v2:
        - The limit is 2 TiB no 5
        - Remove unnecessary sentence in the docs
        - Add missing link
        - Add Michal's reviewed-by

I couldn't find a nice way to document it in SUPPORT.md. So I decided
to only document the restrict in docs/misc/arm/booting.txt for now.

I also couldn't find any way from GRUB/UEFI (I didn't look much) to
specify the loading address.
---
 docs/misc/arm/booting.txt | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/docs/misc/arm/booting.txt b/docs/misc/arm/booting.txt
index 02f7bb65ec6d..547f58a7d981 100644
--- a/docs/misc/arm/booting.txt
+++ b/docs/misc/arm/booting.txt
@@ -21,7 +21,9 @@ The exceptions to this on 32-bit ARM are as follows:
  zImage protocol should still be used and not the stricter "raw
  (non-zImage)" protocol described in arm/Booting.
 
-There are no exception on 64-bit ARM.
+The exceptions to this on 64-bit ARM are as follows:
+
+ Xen binary should be loaded in memory below 2 TiB.
 
 Booting Guests
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-- 
2.40.1