qemu/docs/specs/fw_cfg.txt

QEMU Firmware Configuration (fw_cfg) Device
===========================================

= Guest-side Hardware Interface =

This hardware interface allows the guest to retrieve various data items
(blobs) that can influence how the firmware configures itself, or may
contain tables to be installed for the guest OS. Examples include device
boot order, ACPI and SMBIOS tables, virtual machine UUID, SMP and NUMA
information, kernel/initrd images for direct (Linux) kernel booting, etc.

== Selector (Control) Register ==

* Write only
* Location: platform dependent (IOport or MMIO)
* Width: 16-bit
* Endianness: little-endian (if IOport), or big-endian (if MMIO)

A write to this register sets the index of a firmware configuration
item which can subsequently be accessed via the data register.

Setting the selector register will cause the data offset to be set
to zero. The data offset impacts which data is accessed via the data
register, and is explained below.

Bit14 of the selector register indicates whether the configuration
setting is being written. A value of 0 means the item is only being
read, and all write access to the data port will be ignored. A value
of 1 means the item's data can be overwritten by writes to the data
register. In other words, configuration write mode is enabled when
the selector value is between 0x4000-0x7fff or 0xc000-0xffff.

NOTE: As of QEMU v2.4, writes to the fw_cfg data register are no
      longer supported, and will be ignored (treated as no-ops)!

Bit15 of the selector register indicates whether the configuration
setting is architecture specific. A value of 0 means the item is a
generic configuration item. A value of 1 means the item is specific
to a particular architecture. In other words, generic configuration
items are accessed with a selector value between 0x0000-0x7fff, and
architecture specific configuration items are accessed with a selector
value between 0x8000-0xffff.

== Data Register ==

* Read/Write (writes ignored as of QEMU v2.4)
* Location: platform dependent (IOport [*] or MMIO)
* Width: 8-bit (if IOport), 8/16/32/64-bit (if MMIO)
* Endianness: string-preserving

[*] On platforms where the data register is exposed as an IOport, its
port number will always be one greater than the port number of the
selector register. In other words, the two ports overlap, and can not
be mapped separately.

The data register allows access to an array of bytes for each firmware
configuration data item. The specific item is selected by writing to
the selector register, as described above.

Initially following a write to the selector register, the data offset
will be set to zero. Each successful access to the data register will
increment the data offset by the appropriate access width.

Each firmware configuration item has a maximum length of data
associated with the item. After the data offset has passed the
end of this maximum data length, then any reads will return a data
value of 0x00, and all writes will be ignored.

An N-byte wide read of the data register will return the next available
N bytes of the selected firmware configuration item, as a substring, in
increasing address order, similar to memcpy().

== Register Locations ==

=== x86, x86_64 Register Locations ===

Selector Register IOport: 0x510
Data Register IOport:     0x511

== Firmware Configuration Items ==

=== Signature (Key 0x0000, FW_CFG_SIGNATURE) ===

The presence of the fw_cfg selector and data registers can be verified
by selecting the "signature" item using key 0x0000 (FW_CFG_SIGNATURE),
and reading four bytes from the data register. If the fw_cfg device is
present, the four bytes read will contain the characters "QEMU".

=== Revision (Key 0x0001, FW_CFG_ID) ===

A 32-bit little-endian unsigned int, this item is used as an interface
revision number, and is currently set to 1 by QEMU when fw_cfg is
initialized.

=== File Directory (Key 0x0019, FW_CFG_FILE_DIR) ===

Firmware configuration items stored at selector keys 0x0020 or higher
(FW_CFG_FILE_FIRST or higher) have an associated entry in a directory
structure, which makes it easier for guest-side firmware to identify
and retrieve them. The format of this file directory (from fw_cfg.h in
the QEMU source tree) is shown here, slightly annotated for clarity:

struct FWCfgFiles {		/* the entire file directory fw_cfg item */
    uint32_t count;		/* number of entries, in big-endian format */
    struct FWCfgFile f[];	/* array of file entries, see below */
};

struct FWCfgFile {		/* an individual file entry, 64 bytes total */
    uint32_t size;		/* size of referenced fw_cfg item, big-endian */
    uint16_t select;		/* selector key of fw_cfg item, big-endian */
    uint16_t reserved;
    char name[56];		/* fw_cfg item name, NUL-terminated ascii */
};

=== All Other Data Items ===

Please consult the QEMU source for the most up-to-date and authoritative
list of selector keys and their respective items' purpose and format.

=== Ranges ===

Theoretically, there may be up to 0x4000 generic firmware configuration
items, and up to 0x4000 architecturally specific ones.

Selector Reg.    Range Usage
---------------  -----------
0x0000 - 0x3fff  Generic (0x0000 - 0x3fff, RO)
0x4000 - 0x7fff  Generic (0x0000 - 0x3fff, RW, ignored in QEMU v2.4+)
0x8000 - 0xbfff  Arch. Specific (0x0000 - 0x3fff, RO)
0xc000 - 0xffff  Arch. Specific (0x0000 - 0x3fff, RW, ignored in v2.4+)

In practice, the number of allowed firmware configuration items is given
by the value of FW_CFG_MAX_ENTRY (see fw_cfg.h).

= Host-side API =

The following functions are available to the QEMU programmer for adding
data to a fw_cfg device during guest initialization (see fw_cfg.h for
each function's complete prototype):

== fw_cfg_add_bytes() ==

Given a selector key value, starting pointer, and size, create an item
as a raw "blob" of the given size, available by selecting the given key.
The data referenced by the starting pointer is only linked, NOT copied,
into the data structure of the fw_cfg device.

== fw_cfg_add_string() ==

Instead of a starting pointer and size, this function accepts a pointer
to a NUL-terminated ascii string, and inserts a newly allocated copy of
the string (including the NUL terminator) into the fw_cfg device data
structure.

== fw_cfg_add_iXX() ==

Insert an XX-bit item, where XX may be 16, 32, or 64. These functions
will convert a 16-, 32-, or 64-bit integer to little-endian, then add
a dynamically allocated copy of the appropriately sized item to fw_cfg
under the given selector key value.

== fw_cfg_add_file() ==

Given a filename (i.e., fw_cfg item name), starting pointer, and size,
create an item as a raw "blob" of the given size. Unlike fw_cfg_add_bytes()
above, the next available selector key (above 0x0020, FW_CFG_FILE_FIRST)
will be used, and a new entry will be added to the file directory structure
(at key 0x0019), containing the item name, blob size, and automatically
assigned selector key value. The data referenced by the starting pointer
is only linked, NOT copied, into the fw_cfg data structure.

== fw_cfg_add_file_callback() ==

Like fw_cfg_add_file(), but additionally sets pointers to a callback
function (and opaque argument), which will be executed host-side by
QEMU each time a byte is read by the guest from this particular item.

NOTE: The callback function is given the opaque argument set by
fw_cfg_add_file_callback(), but also the current data offset,
allowing it the option of only acting upon specific offset values
(e.g., 0, before the first data byte of the selected item is
returned to the guest).

== fw_cfg_modify_file() ==

Given a filename (i.e., fw_cfg item name), starting pointer, and size,
completely replace the configuration item referenced by the given item
name with the new given blob. If an existing blob is found, its
callback information is removed, and a pointer to the old data is
returned to allow the caller to free it, helping avoid memory leaks.
If a configuration item does not already exist under the given item
name, a new item will be created as with fw_cfg_add_file(), and NULL
is returned to the caller. In any case, the data referenced by the
starting pointer is only linked, NOT copied, into the fw_cfg data
structure.

== fw_cfg_add_callback() ==

Like fw_cfg_add_bytes(), but additionally sets pointers to a callback
function (and opaque argument), which will be executed host-side by
QEMU each time a guest-side write operation to this particular item
completes fully overwriting the item's data.

NOTE: This function is deprecated, and will be completely removed
starting with QEMU v2.4.

== Externally Provided Items ==

As of v2.4, "file" fw_cfg items (i.e., items with selector keys above
FW_CFG_FILE_FIRST, and with a corresponding entry in the fw_cfg file
directory structure) may be inserted via the QEMU command line, using
the following syntax:

    -fw_cfg [name=]<item_name>,file=<path>

where <item_name> is the fw_cfg item name, and <path> is the location
on the host file system of a file containing the data to be inserted.

NOTE: Users *SHOULD* choose item names beginning with the prefix "opt/"
when using the "-fw_cfg" command line option, to avoid conflicting with
item names used internally by QEMU. For instance:

    -fw_cfg name=opt/my_item_name,file=./my_blob.bin

Similarly, QEMU developers *SHOULD NOT* use item names prefixed with
"opt/" when inserting items programmatically, e.g. via fw_cfg_add_file().