1What:		/sys/firmware/qemu_fw_cfg/
2Date:		August 2015
3Contact:	Gabriel Somlo <somlo@cmu.edu>
4Description:
5		Several different architectures supported by QEMU (x86, arm,
6		sun4*, ppc/mac) are provisioned with a firmware configuration
7		(fw_cfg) device, originally intended as a way for the host to
8		provide configuration data to the guest firmware. Starting
9		with QEMU v2.4, arbitrary fw_cfg file entries may be specified
10		by the user on the command line, which makes fw_cfg additionally
11		useful as an out-of-band, asynchronous mechanism for providing
12		configuration data to the guest userspace.
13
14		The authoritative guest-side hardware interface documentation
15		to the fw_cfg device can be found in "docs/specs/fw_cfg.txt"
16		in the QEMU source tree.
17
18		**SysFS fw_cfg Interface**
19
20		The fw_cfg sysfs interface described in this document is only
21		intended to display discoverable blobs (i.e., those registered
22		with the file directory), as there is no way to determine the
23		presence or size of "legacy" blobs (with selector keys between
24		0x0002 and 0x0018) programmatically.
25
26		All fw_cfg information is shown under:
27
28			/sys/firmware/qemu_fw_cfg/
29
30		The only legacy blob displayed is the fw_cfg device revision:
31
32			/sys/firmware/qemu_fw_cfg/rev
33
34		**Discoverable fw_cfg blobs by selector key**
35
36		All discoverable blobs listed in the fw_cfg file directory are
37		displayed as entries named after their unique selector key
38		value, e.g.:
39
40			/sys/firmware/qemu_fw_cfg/by_key/32
41			/sys/firmware/qemu_fw_cfg/by_key/33
42			/sys/firmware/qemu_fw_cfg/by_key/34
43			...
44
45		Each such fw_cfg sysfs entry has the following values exported
46		as attributes:
47
48		====	  ====================================================
49		name	  The 56-byte nul-terminated ASCII string used as the
50			  blob's 'file name' in the fw_cfg directory.
51		size	  The length of the blob, as given in the fw_cfg
52			  directory.
53		key	  The value of the blob's selector key as given in the
54			  fw_cfg directory. This value is the same as used in
55			  the parent directory name.
56		raw	  The raw bytes of the blob, obtained by selecting the
57			  entry via the control register, and reading a number
58			  of bytes equal to the blob size from the data
59			  register.
60		====	  ====================================================
61
62		**Listing fw_cfg blobs by file name**
63
64		While the fw_cfg device does not impose any specific naming
65		convention on the blobs registered in the file directory,
66		QEMU developers have traditionally used path name semantics
67		to give each blob a descriptive name. For example::
68
69			"bootorder"
70			"genroms/kvmvapic.bin"
71			"etc/e820"
72			"etc/boot-fail-wait"
73			"etc/system-states"
74			"etc/table-loader"
75			"etc/acpi/rsdp"
76			"etc/acpi/tables"
77			"etc/smbios/smbios-tables"
78			"etc/smbios/smbios-anchor"
79			...
80
81		In addition to the listing by unique selector key described
82		above, the fw_cfg sysfs driver also attempts to build a tree
83		of directories matching the path name components of fw_cfg
84		blob names, ending in symlinks to the by_key entry for each
85		"basename", as illustrated below (assume current directory is
86		/sys/firmware)::
87
88		    qemu_fw_cfg/by_name/bootorder -> ../by_key/38
89		    qemu_fw_cfg/by_name/etc/e820 -> ../../by_key/35
90		    qemu_fw_cfg/by_name/etc/acpi/rsdp -> ../../../by_key/41
91		    ...
92
93		Construction of the directory tree and symlinks is done on a
94		"best-effort" basis, as there is no guarantee that components
95		of fw_cfg blob names are always "well behaved". I.e., there is
96		the possibility that a symlink (basename) will conflict with
97		a dirname component of another fw_cfg blob, in which case the
98		creation of the offending /sys/firmware/qemu_fw_cfg/by_name
99		entry will be skipped.
100
101		The authoritative list of entries will continue to be found
102		under the /sys/firmware/qemu_fw_cfg/by_key directory.
103