100 lines
3.6 KiB
Text
100 lines
3.6 KiB
Text
What: /sys/firmware/qemu_fw_cfg/
|
|
Date: August 2015
|
|
Contact: Gabriel Somlo <somlo@cmu.edu>
|
|
Description:
|
|
Several different architectures supported by QEMU (x86, arm,
|
|
sun4*, ppc/mac) are provisioned with a firmware configuration
|
|
(fw_cfg) device, originally intended as a way for the host to
|
|
provide configuration data to the guest firmware. Starting
|
|
with QEMU v2.4, arbitrary fw_cfg file entries may be specified
|
|
by the user on the command line, which makes fw_cfg additionally
|
|
useful as an out-of-band, asynchronous mechanism for providing
|
|
configuration data to the guest userspace.
|
|
|
|
The authoritative guest-side hardware interface documentation
|
|
to the fw_cfg device can be found in "docs/specs/fw_cfg.txt"
|
|
in the QEMU source tree.
|
|
|
|
=== SysFS fw_cfg Interface ===
|
|
|
|
The fw_cfg sysfs interface described in this document is only
|
|
intended to display discoverable blobs (i.e., those registered
|
|
with the file directory), as there is no way to determine the
|
|
presence or size of "legacy" blobs (with selector keys between
|
|
0x0002 and 0x0018) programmatically.
|
|
|
|
All fw_cfg information is shown under:
|
|
|
|
/sys/firmware/qemu_fw_cfg/
|
|
|
|
The only legacy blob displayed is the fw_cfg device revision:
|
|
|
|
/sys/firmware/qemu_fw_cfg/rev
|
|
|
|
--- Discoverable fw_cfg blobs by selector key ---
|
|
|
|
All discoverable blobs listed in the fw_cfg file directory are
|
|
displayed as entries named after their unique selector key
|
|
value, e.g.:
|
|
|
|
/sys/firmware/qemu_fw_cfg/by_key/32
|
|
/sys/firmware/qemu_fw_cfg/by_key/33
|
|
/sys/firmware/qemu_fw_cfg/by_key/34
|
|
...
|
|
|
|
Each such fw_cfg sysfs entry has the following values exported
|
|
as attributes:
|
|
|
|
name : The 56-byte nul-terminated ASCII string used as the
|
|
blob's 'file name' in the fw_cfg directory.
|
|
size : The length of the blob, as given in the fw_cfg
|
|
directory.
|
|
key : The value of the blob's selector key as given in the
|
|
fw_cfg directory. This value is the same as used in
|
|
the parent directory name.
|
|
raw : The raw bytes of the blob, obtained by selecting the
|
|
entry via the control register, and reading a number
|
|
of bytes equal to the blob size from the data
|
|
register.
|
|
|
|
--- Listing fw_cfg blobs by file name ---
|
|
|
|
While the fw_cfg device does not impose any specific naming
|
|
convention on the blobs registered in the file directory,
|
|
QEMU developers have traditionally used path name semantics
|
|
to give each blob a descriptive name. For example:
|
|
|
|
"bootorder"
|
|
"genroms/kvmvapic.bin"
|
|
"etc/e820"
|
|
"etc/boot-fail-wait"
|
|
"etc/system-states"
|
|
"etc/table-loader"
|
|
"etc/acpi/rsdp"
|
|
"etc/acpi/tables"
|
|
"etc/smbios/smbios-tables"
|
|
"etc/smbios/smbios-anchor"
|
|
...
|
|
|
|
In addition to the listing by unique selector key described
|
|
above, the fw_cfg sysfs driver also attempts to build a tree
|
|
of directories matching the path name components of fw_cfg
|
|
blob names, ending in symlinks to the by_key entry for each
|
|
"basename", as illustrated below (assume current directory is
|
|
/sys/firmware):
|
|
|
|
qemu_fw_cfg/by_name/bootorder -> ../by_key/38
|
|
qemu_fw_cfg/by_name/etc/e820 -> ../../by_key/35
|
|
qemu_fw_cfg/by_name/etc/acpi/rsdp -> ../../../by_key/41
|
|
...
|
|
|
|
Construction of the directory tree and symlinks is done on a
|
|
"best-effort" basis, as there is no guarantee that components
|
|
of fw_cfg blob names are always "well behaved". I.e., there is
|
|
the possibility that a symlink (basename) will conflict with
|
|
a dirname component of another fw_cfg blob, in which case the
|
|
creation of the offending /sys/firmware/qemu_fw_cfg/by_name
|
|
entry will be skipped.
|
|
|
|
The authoritative list of entries will continue to be found
|
|
under the /sys/firmware/qemu_fw_cfg/by_key directory.
|