docs: Add a generic loader explanation document

Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 9d991a2df990cf55e2630410a5a03ea48930af5d.1475195078.git.alistair.francis@xilinx.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

diff --git a/docs/generic-loader.txt b/docs/generic-loader.txt
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+Copyright (c) 2016 Xilinx Inc.
+
+This work is licensed under the terms of the GNU GPL, version 2 or later.  See
+the COPYING file in the top-level directory.
+
+
+The 'loader' device allows the user to load multiple images or values into
+QEMU at startup.
+
+Loading Data into Memory Values
+---------------------
+The loader device allows memory values to be set from the command line. This
+can be done by following the syntax below:
+
+     -device loader,addr=<addr>,data=<data>,data-len=<data-len>
+                   [,data-be=<data-be>][,cpu-num=<cpu-num>]
+
+    <addr>      - The address to store the data in.
+    <data>      - The value to be written to the address. The maximum size of
+                  the data is 8 bytes.
+    <data-len>  - The length of the data in bytes. This argument must be
+                  included if the data argument is.
+    <data-be>   - Set to true if the data to be stored on the guest should be
+                  written as big endian data. The default is to write little
+                  endian data.
+    <cpu-num>   - The number of the CPU's address space where the data should
+                  be loaded. If not specified the address space of the first
+                  CPU is used.
+
+All values are parsed using the standard QemuOps parsing. This allows the user
+to specify any values in any format supported. By default the values
+will be parsed as decimal. To use hex values the user should prefix the number
+with a '0x'.
+
+An example of loading value 0x8000000e to address 0xfd1a0104 is:
+    -device loader,addr=0xfd1a0104,data=0x8000000e,data-len=4
+
+Setting a CPU's Program Counter
+---------------------
+The loader device allows the CPU's PC to be set from the command line. This
+can be done by following the syntax below:
+
+     -device loader,addr=<addr>,cpu-num=<cpu-num>
+
+    <addr>      - The value to use as the CPU's PC.
+    <cpu-num>   - The number of the CPU whose PC should be set to the
+                  specified value.
+
+All values are parsed using the standard QemuOps parsing. This allows the user
+to specify any values in any format supported. By default the values
+will be parsed as decimal. To use hex values the user should prefix the number
+with a '0x'.
+
+An example of setting CPU 0's PC to 0x8000 is:
+    -device loader,addr=0x8000,cpu-num=0
+
+Loading Files
+---------------------
+The loader device also allows files to be loaded into memory. This can be done
+similarly to setting memory values. The syntax is shown below:
+
+    -device loader,file=<file>[,addr=<addr>][,cpu-num=<cpu-num>][,force-raw=<raw>]
+
+    <file>      - A file to be loaded into memory
+    <addr>      - The addr in memory that the file should be loaded. This is
+                  ignored if you are using an ELF (unless force-raw is true).
+                  This is required if you aren't loading an ELF.
+    <cpu-num>   - This specifies the CPU that should be used. This is an
+                  optional argument and will cause the CPU's PC to be set to
+                  where the image is stored or in the case of an ELF file to
+                  the value in the header. This option should only be used
+                  for the boot image.
+                  This will also cause the image to be written to the specified
+                  CPU's address space. If not specified, the default is CPU 0.
+    <force-raw> - Forces the file to be treated as a raw image. This can be
+                  used to specify the load address of ELF files.
+
+All values are parsed using the standard QemuOps parsing. This allows the user
+to specify any values in any format supported. By default the values
+will be parsed as decimal. To use hex values the user should prefix the number
+with a '0x'.
+
+An example of loading an ELF file which CPU0 will boot is shown below:
+    -device loader,file=./images/boot.elf,cpu-num=0