docs: add some documentation for the guest-loader

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20210303173642.3805-7-alex.bennee@linaro.org>

MAINTAINERS

@@ -2026,6 +2026,7 @@ Guest Loader
 M: Alex Bennée <alex.bennee@linaro.org>
 S: Maintained
 F: hw/core/guest-loader.c
+F: docs/system/guest-loader.rst
 
 Intel Hexadecimal Object File Loader
 M: Su Hang <suhang16@mails.ucas.ac.cn>

docs/system/guest-loader.rst

@@ -0,0 +1,54 @@
+..
+   Copyright (c) 2020, Linaro
+
+Guest Loader
+------------
+
+The guest loader is similar to the `generic-loader` although it is
+aimed at a particular use case of loading hypervisor guests. This is
+useful for debugging hypervisors without having to jump through the
+hoops of firmware and boot-loaders.
+
+The guest loader does two things:
+
+  - load blobs (kernels and initial ram disks) into memory
+  - sets platform FDT data so hypervisors can find and boot them
+
+This is what is typically done by a boot-loader like grub using it's
+multi-boot capability. A typical example would look like:
+
+.. parsed-literal::
+
+  |qemu_system| -kernel ~/xen.git/xen/xen \
+    -append "dom0_mem=1G,max:1G loglvl=all guest_loglvl=all" \
+    -device guest-loader,addr=0x42000000,kernel=Image,bootargs="root=/dev/sda2 ro console=hvc0 earlyprintk=xen" \
+    -device guest-loader,addr=0x47000000,initrd=rootfs.cpio
+
+In the above example the Xen hypervisor is loaded by the -kernel
+parameter and passed it's boot arguments via -append. The Dom0 guest
+is loaded into the areas of memory. Each blob will get
+`/chosen/module@<addr>` entry in the FDT to indicate it's location and
+size. Additional information can be passed with by using additional
+arguments.
+
+Currently the only supported machines which use FDT data to boot are
+the ARM and RiscV `virt` machines.
+
+Arguments
+^^^^^^^^^
+
+The full syntax of the guest-loader is::
+
+  -device guest-loader,addr=<addr>[,kernel=<file>,[bootargs=<args>]][,initrd=<file>]
+
+``addr=<addr>``
+  This is mandatory and indicates the start address of the blob.
+
+``kernel|initrd=<file>``
+  Indicates the filename of the kernel or initrd blob. Both blobs will
+  have the "multiboot,module" compatibility string as well as
+  "multiboot,kernel" or "multiboot,ramdisk" as appropriate.
+
+``bootargs=<args>``
+  This is an optional field for kernel blobs which will pass command
+  like via the `/chosen/module@<addr>/bootargs` node.

docs/system/index.rst

@@ -26,6 +26,7 @@ Contents:
    ivshmem
    linuxboot
    generic-loader
+   guest-loader
    vnc-security
    tls
    gdb