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1What: /sys/firmware/qemu_fw_cfg/2Date: August 20153Contact: Gabriel Somlo <somlo@cmu.edu>4Description:5 Several different architectures supported by QEMU (x86, arm,6 sun4*, ppc/mac) are provisioned with a firmware configuration7 (fw_cfg) device, originally intended as a way for the host to8 provide configuration data to the guest firmware. Starting9 with QEMU v2.4, arbitrary fw_cfg file entries may be specified10 by the user on the command line, which makes fw_cfg additionally11 useful as an out-of-band, asynchronous mechanism for providing12 configuration data to the guest userspace.13 14 The authoritative guest-side hardware interface documentation15 to the fw_cfg device can be found in "docs/specs/fw_cfg.rst"16 in the QEMU source tree, or online at:17 https://qemu-project.gitlab.io/qemu/specs/fw_cfg.html18 19 **SysFS fw_cfg Interface**20 21 The fw_cfg sysfs interface described in this document is only22 intended to display discoverable blobs (i.e., those registered23 with the file directory), as there is no way to determine the24 presence or size of "legacy" blobs (with selector keys between25 0x0002 and 0x0018) programmatically.26 27 All fw_cfg information is shown under:28 29 /sys/firmware/qemu_fw_cfg/30 31 The only legacy blob displayed is the fw_cfg device revision:32 33 /sys/firmware/qemu_fw_cfg/rev34 35 **Discoverable fw_cfg blobs by selector key**36 37 All discoverable blobs listed in the fw_cfg file directory are38 displayed as entries named after their unique selector key39 value, e.g.:40 41 /sys/firmware/qemu_fw_cfg/by_key/3242 /sys/firmware/qemu_fw_cfg/by_key/3343 /sys/firmware/qemu_fw_cfg/by_key/3444 ...45 46 Each such fw_cfg sysfs entry has the following values exported47 as attributes:48 49 ==== ====================================================50 name The 56-byte nul-terminated ASCII string used as the51 blob's 'file name' in the fw_cfg directory.52 size The length of the blob, as given in the fw_cfg53 directory.54 key The value of the blob's selector key as given in the55 fw_cfg directory. This value is the same as used in56 the parent directory name.57 raw The raw bytes of the blob, obtained by selecting the58 entry via the control register, and reading a number59 of bytes equal to the blob size from the data60 register.61 ==== ====================================================62 63 **Listing fw_cfg blobs by file name**64 65 While the fw_cfg device does not impose any specific naming66 convention on the blobs registered in the file directory,67 QEMU developers have traditionally used path name semantics68 to give each blob a descriptive name. For example::69 70 "bootorder"71 "genroms/kvmvapic.bin"72 "etc/e820"73 "etc/boot-fail-wait"74 "etc/system-states"75 "etc/table-loader"76 "etc/acpi/rsdp"77 "etc/acpi/tables"78 "etc/smbios/smbios-tables"79 "etc/smbios/smbios-anchor"80 ...81 82 In addition to the listing by unique selector key described83 above, the fw_cfg sysfs driver also attempts to build a tree84 of directories matching the path name components of fw_cfg85 blob names, ending in symlinks to the by_key entry for each86 "basename", as illustrated below (assume current directory is87 /sys/firmware)::88 89 qemu_fw_cfg/by_name/bootorder -> ../by_key/3890 qemu_fw_cfg/by_name/etc/e820 -> ../../by_key/3591 qemu_fw_cfg/by_name/etc/acpi/rsdp -> ../../../by_key/4192 ...93 94 Construction of the directory tree and symlinks is done on a95 "best-effort" basis, as there is no guarantee that components96 of fw_cfg blob names are always "well behaved". I.e., there is97 the possibility that a symlink (basename) will conflict with98 a dirname component of another fw_cfg blob, in which case the99 creation of the offending /sys/firmware/qemu_fw_cfg/by_name100 entry will be skipped.101 102 The authoritative list of entries will continue to be found103 under the /sys/firmware/qemu_fw_cfg/by_key directory.104