[PATCH for-4.19] 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 5 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

Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>

----

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, 4 insertions(+)

diff --git a/docs/misc/arm/booting.txt b/docs/misc/arm/booting.txt
index 02f7bb65ec6d..c6bdeafe5e5b 100644
--- a/docs/misc/arm/booting.txt
+++ b/docs/misc/arm/booting.txt
@@ -21,6 +21,10 @@ 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.
 
+The exceptions to this on 64-bit ARM are as follows:
+
+ Xen binary should be loaded in memory below 2 TiB.
+
 There are no exception on 64-bit ARM.
 
 Booting Guests
-- 
2.40.1