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