docs: add Arm FF-A mediator

Describes a FF-A version 1.1 [1] mediator to communicate with a Secure
Partition in secure world.

[1] https://developer.arm.com/documentation/den0077/latest

Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Henry Wang <Henry.Wang@arm.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>

CHANGELOG.md

@@ -24,6 +24,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
  - xl/libxl can customize SMBIOS strings for HVM guests.
  - Add support for AVX512-FP16 on x86.
  - On Arm, Xen supports guests running SVE/SVE2 instructions. (Tech Preview)
+ - On Arm, add suport for Firmware Framework for Arm A-profile (FF-A) Mediator
+   (Tech Preview)
 
 ### Removed
  - On x86, the "pku" command line option has been removed.  It has never

SUPPORT.md

@@ -842,6 +842,15 @@ that covers the DMA of the device to be passed through.
 
 No support for QEMU backends in a 16K or 64K domain.
 
+### ARM: Firmware Framework for Arm A-profile (FF-A) Mediator
+
+    Status, Arm64: Tech Preview
+
+There are still some code paths where a vCPU may hog a pCPU longer than
+necessary. The FF-A mediator is not yet implemented for Arm32. Part of the
+FF-A specification is not supported, see the top comment in
+xen/arch/arm/tee/ffa.c for limitations.
+
 ### ARM: Guest Device Tree support
 
     Status: Supported

docs/man/xl.cfg.5.pod.in

@@ -1656,6 +1656,21 @@ in OP-TEE.
 
 This feature is a B<technology preview>.
 
+=item B<ffa>
+
+B<Arm only.> Allow a guest to communicate via FF-A with Secure Partitions
+(SP), default false.
+
+Currently only a small subset of the FF-A specification is supported. Just
+enough to communicate with OP-TEE. In general only direct messaging and
+sharing memory with one SP. More advanced use cases where memory might be
+shared or donated to multiple SPs are not supported.
+
+See L<https://developer.arm.com/documentation/den0077/latest> for more
+information about FF-A.
+
+This feature is a B<technology preview>.
+
 =back
 
 =back