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Diffstat (limited to 'Documentation/ABI')
| -rw-r--r-- | Documentation/ABI/testing/sysfs-firmware-qemu_fw_cfg | 100 |
1 files changed, 100 insertions, 0 deletions
diff --git a/Documentation/ABI/testing/sysfs-firmware-qemu_fw_cfg b/Documentation/ABI/testing/sysfs-firmware-qemu_fw_cfg new file mode 100644 index 000000000000..011dda4f8e8a --- /dev/null +++ b/Documentation/ABI/testing/sysfs-firmware-qemu_fw_cfg @@ -0,0 +1,100 @@ +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. |