xen: arm: doc: Add firmware requirements

Add a section about what the firmware should do in EL3 before starting Xen.

E.g guest will use HVC instruction to issue hypercall. As this can be set only
at EL3, i.e. outside Xen, document this boot requirement.

Signed-off-by: Dirk Behme <dirk.behme@de.bosch.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

diff --git a/docs/misc/arm/booting.txt b/docs/misc/arm/booting.txt
index 9802e5e20fd8c7da94eaa7b639b7530b951760eb..c7c1d7ea58429a711f24770dfccfa9c50a29fbaa 100644 (file)
--- a/docs/misc/arm/booting.txt
+++ b/docs/misc/arm/booting.txt
@@ -23,6 +23,17 @@ The exceptions to this on 32-bit ARM are as follows:
 
 There are no exception on 64-bit ARM.
 
+
+Firmware/bootloader requirements
+--------------------------------
+
+Xen relies on some settings the firmware has to configure in EL3 before starting Xen.
+
+* Xen must be entered in NS EL2 mode
+
+* The bit SCR_EL3.HCR (resp. SCR.HCE for 32-bit ARM) must be set to 1.
+
+
 [1] linux/Documentation/arm/Booting
 Latest version: http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/Documentation/arm/Booting