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diff --git a/docs/specs/fw_cfg.txt b/docs/specs/fw_cfg.txt new file mode 100644 index 0000000..6accd92 --- /dev/null +++ b/docs/specs/fw_cfg.txt @@ -0,0 +1,205 @@ +QEMU Firmware Configuration (fw_cfg) Device +=========================================== + += Guest-side Hardware Interface = + +This hardware interface allows the guest to retrieve various data items +(blobs) that can influence how the firmware configures itself, or may +contain tables to be installed for the guest OS. Examples include device +boot order, ACPI and SMBIOS tables, virtual machine UUID, SMP and NUMA +information, kernel/initrd images for direct (Linux) kernel booting, etc. + +== Selector (Control) Register == + +* Write only +* Location: platform dependent (IOport or MMIO) +* Width: 16-bit +* Endianness: little-endian (if IOport), or big-endian (if MMIO) + +A write to this register sets the index of a firmware configuration +item which can subsequently be accessed via the data register. + +Setting the selector register will cause the data offset to be set +to zero. The data offset impacts which data is accessed via the data +register, and is explained below. + +Bit14 of the selector register indicates whether the configuration +setting is being written. A value of 0 means the item is only being +read, and all write access to the data port will be ignored. A value +of 1 means the item's data can be overwritten by writes to the data +register. In other words, configuration write mode is enabled when +the selector value is between 0x4000-0x7fff or 0xc000-0xffff. + +NOTE: As of QEMU v2.4, writes to the fw_cfg data register are no + longer supported, and will be ignored (treated as no-ops)! + +Bit15 of the selector register indicates whether the configuration +setting is architecture specific. A value of 0 means the item is a +generic configuration item. A value of 1 means the item is specific +to a particular architecture. In other words, generic configuration +items are accessed with a selector value between 0x0000-0x7fff, and +architecture specific configuration items are accessed with a selector +value between 0x8000-0xffff. + +== Data Register == + +* Read/Write (writes ignored as of QEMU v2.4) +* Location: platform dependent (IOport [*] or MMIO) +* Width: 8-bit (if IOport), 8/16/32/64-bit (if MMIO) +* Endianness: string-preserving + +[*] On platforms where the data register is exposed as an IOport, its +port number will always be one greater than the port number of the +selector register. In other words, the two ports overlap, and can not +be mapped separately. + +The data register allows access to an array of bytes for each firmware +configuration data item. The specific item is selected by writing to +the selector register, as described above. + +Initially following a write to the selector register, the data offset +will be set to zero. Each successful access to the data register will +increment the data offset by the appropriate access width. + +Each firmware configuration item has a maximum length of data +associated with the item. After the data offset has passed the +end of this maximum data length, then any reads will return a data +value of 0x00, and all writes will be ignored. + +An N-byte wide read of the data register will return the next available +N bytes of the selected firmware configuration item, as a substring, in +increasing address order, similar to memcpy(). + +== Register Locations == + +=== x86, x86_64 Register Locations === + +Selector Register IOport: 0x510 +Data Register IOport: 0x511 + +== Firmware Configuration Items == + +=== Signature (Key 0x0000, FW_CFG_SIGNATURE) === + +The presence of the fw_cfg selector and data registers can be verified +by selecting the "signature" item using key 0x0000 (FW_CFG_SIGNATURE), +and reading four bytes from the data register. If the fw_cfg device is +present, the four bytes read will contain the characters "QEMU". + +=== Revision (Key 0x0001, FW_CFG_ID) === + +A 32-bit little-endian unsigned int, this item is used as an interface +revision number, and is currently set to 1 by QEMU when fw_cfg is +initialized. + +=== File Directory (Key 0x0019, FW_CFG_FILE_DIR) === + +Firmware configuration items stored at selector keys 0x0020 or higher +(FW_CFG_FILE_FIRST or higher) have an associated entry in a directory +structure, which makes it easier for guest-side firmware to identify +and retrieve them. The format of this file directory (from fw_cfg.h in +the QEMU source tree) is shown here, slightly annotated for clarity: + +struct FWCfgFiles { /* the entire file directory fw_cfg item */ + uint32_t count; /* number of entries, in big-endian format */ + struct FWCfgFile f[]; /* array of file entries, see below */ +}; + +struct FWCfgFile { /* an individual file entry, 64 bytes total */ + uint32_t size; /* size of referenced fw_cfg item, big-endian */ + uint16_t select; /* selector key of fw_cfg item, big-endian */ + uint16_t reserved; + char name[56]; /* fw_cfg item name, NUL-terminated ascii */ +}; + +=== All Other Data Items === + +Please consult the QEMU source for the most up-to-date and authoritative +list of selector keys and their respective items' purpose and format. + +=== Ranges === + +Theoretically, there may be up to 0x4000 generic firmware configuration +items, and up to 0x4000 architecturally specific ones. + +Selector Reg. Range Usage +--------------- ----------- +0x0000 - 0x3fff Generic (0x0000 - 0x3fff, RO) +0x4000 - 0x7fff Generic (0x0000 - 0x3fff, RW, ignored in QEMU v2.4+) +0x8000 - 0xbfff Arch. Specific (0x0000 - 0x3fff, RO) +0xc000 - 0xffff Arch. Specific (0x0000 - 0x3fff, RW, ignored in v2.4+) + +In practice, the number of allowed firmware configuration items is given +by the value of FW_CFG_MAX_ENTRY (see fw_cfg.h). + += Host-side API = + +The following functions are available to the QEMU programmer for adding +data to a fw_cfg device during guest initialization (see fw_cfg.h for +each function's complete prototype): + +== fw_cfg_add_bytes() == + +Given a selector key value, starting pointer, and size, create an item +as a raw "blob" of the given size, available by selecting the given key. +The data referenced by the starting pointer is only linked, NOT copied, +into the data structure of the fw_cfg device. + +== fw_cfg_add_string() == + +Instead of a starting pointer and size, this function accepts a pointer +to a NUL-terminated ascii string, and inserts a newly allocated copy of +the string (including the NUL terminator) into the fw_cfg device data +structure. + +== fw_cfg_add_iXX() == + +Insert an XX-bit item, where XX may be 16, 32, or 64. These functions +will convert a 16-, 32-, or 64-bit integer to little-endian, then add +a dynamically allocated copy of the appropriately sized item to fw_cfg +under the given selector key value. + +== fw_cfg_add_file() == + +Given a filename (i.e., fw_cfg item name), starting pointer, and size, +create an item as a raw "blob" of the given size. Unlike fw_cfg_add_bytes() +above, the next available selector key (above 0x0020, FW_CFG_FILE_FIRST) +will be used, and a new entry will be added to the file directory structure +(at key 0x0019), containing the item name, blob size, and automatically +assigned selector key value. The data referenced by the starting pointer +is only linked, NOT copied, into the fw_cfg data structure. + +== fw_cfg_add_file_callback() == + +Like fw_cfg_add_file(), but additionally sets pointers to a callback +function (and opaque argument), which will be executed host-side by +QEMU each time a byte is read by the guest from this particular item. + +NOTE: The callback function is given the opaque argument set by +fw_cfg_add_file_callback(), but also the current data offset, +allowing it the option of only acting upon specific offset values +(e.g., 0, before the first data byte of the selected item is +returned to the guest). + +== fw_cfg_modify_file() == + +Given a filename (i.e., fw_cfg item name), starting pointer, and size, +completely replace the configuration item referenced by the given item +name with the new given blob. If an existing blob is found, its +callback information is removed, and a pointer to the old data is +returned to allow the caller to free it, helping avoid memory leaks. +If a configuration item does not already exist under the given item +name, a new item will be created as with fw_cfg_add_file(), and NULL +is returned to the caller. In any case, the data referenced by the +starting pointer is only linked, NOT copied, into the fw_cfg data +structure. + +== fw_cfg_add_callback() == + +Like fw_cfg_add_bytes(), but additionally sets pointers to a callback +function (and opaque argument), which will be executed host-side by +QEMU each time a guest-side write operation to this particular item +completes fully overwriting the item's data. + +NOTE: This function is deprecated, and will be completely removed +starting with QEMU v2.4. |
