--- /dev/null
+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)!
+
+NOTE: As of QEMU v2.9, writes are reinstated, but only through the DMA
+ interface (see below). Furthermore, writeability of any specific item is
+ governed independently of Bit14 in the selector key value.
+
+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, but see the DMA interface)
+* 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
+DMA Address IOport: 0x514
+
+=== Arm Register Locations ===
+
+Selector Register address: Base + 8 (2 bytes)
+Data Register address: Base + 0 (8 bytes)
+DMA Address address: Base + 16 (8 bytes)
+
+== ACPI Interface ==
+
+The fw_cfg device is defined with ACPI ID "QEMU0002". Since we expect
+ACPI tables to be passed into the guest through the fw_cfg device itself,
+the guest-side firmware can not use ACPI to find fw_cfg. However, once the
+firmware is finished setting up ACPI tables and hands control over to the
+guest kernel, the latter can use the fw_cfg ACPI node for a more accurate
+inventory of in-use IOport or MMIO regions.
+
+== 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".
+
+If the DMA interface is available, then reading the DMA Address
+Register returns 0x51454d5520434647 ("QEMU CFG" in big-endian format).
+
+=== Revision / feature bitmap (Key 0x0001, FW_CFG_ID) ===
+
+A 32-bit little-endian unsigned int, this item is used to check for enabled
+features.
+ - Bit 0: traditional interface. Always set.
+ - Bit 1: DMA interface.
+
+=== 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, format and writeability.
+
+=== 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, generally RO, possibly RW through
+ the DMA interface in QEMU v2.9+)
+0x4000 - 0x7fff Generic (0x0000 - 0x3fff, RW, ignored in QEMU v2.4+)
+0x8000 - 0xbfff Arch. Specific (0x0000 - 0x3fff, generally RO, possibly RW
+ through the DMA interface in QEMU v2.9+)
+0xc000 - 0xffff Arch. Specific (0x0000 - 0x3fff, RW, ignored in v2.4+)
+
+In practice, the number of allowed firmware configuration items depends on the
+machine type/version.
+
+= Guest-side DMA Interface =
+
+If bit 1 of the feature bitmap is set, the DMA interface is present. This does
+not replace the existing fw_cfg interface, it is an add-on. This interface
+can be used through the 64-bit wide address register.
+
+The address register is in big-endian format. The value for the register is 0
+at startup and after an operation. A write to the least significant half (at
+offset 4) triggers an operation. This means that operations with 32-bit
+addresses can be triggered with just one write, whereas operations with
+64-bit addresses can be triggered with one 64-bit write or two 32-bit writes,
+starting with the most significant half (at offset 0).
+
+In this register, the physical address of a FWCfgDmaAccess structure in RAM
+should be written. This is the format of the FWCfgDmaAccess structure:
+
+typedef struct FWCfgDmaAccess {
+ uint32_t control;
+ uint32_t length;
+ uint64_t address;
+} FWCfgDmaAccess;
+
+The fields of the structure are in big endian mode, and the field at the lowest
+address is the "control" field.
+
+The "control" field has the following bits:
+ - Bit 0: Error
+ - Bit 1: Read
+ - Bit 2: Skip
+ - Bit 3: Select. The upper 16 bits are the selected index.
+ - Bit 4: Write
+
+When an operation is triggered, if the "control" field has bit 3 set, the
+upper 16 bits are interpreted as an index of a firmware configuration item.
+This has the same effect as writing the selector register.
+
+If the "control" field has bit 1 set, a read operation will be performed.
+"length" bytes for the current selector and offset will be copied into the
+physical RAM address specified by the "address" field.
+
+If the "control" field has bit 4 set (and not bit 1), a write operation will be
+performed. "length" bytes will be copied from the physical RAM address
+specified by the "address" field to the current selector and offset. QEMU
+prevents starting or finishing the write beyond the end of the item associated
+with the current selector (i.e., the item cannot be resized). Truncated writes
+are dropped entirely. Writes to read-only items are also rejected. All of these
+write errors set bit 0 (the error bit) in the "control" field.
+
+If the "control" field has bit 2 set (and neither bit 1 nor bit 4), a skip
+operation will be performed. The offset for the current selector will be
+advanced "length" bytes.
+
+To check the result, read the "control" field:
+ error bit set -> something went wrong.
+ all bits cleared -> transfer finished successfully.
+ otherwise -> transfer still in progress (doesn't happen
+ today due to implementation not being async,
+ but may in the future).
+
+= Externally Provided Items =
+
+Since v2.4, "file" fw_cfg items (i.e., items with selector keys above
+FW_CFG_FILE_FIRST, and with a corresponding entry in the fw_cfg file
+directory structure) may be inserted via the QEMU command line, using
+the following syntax:
+
+ -fw_cfg [name=]<item_name>,file=<path>
+
+Or
+
+ -fw_cfg [name=]<item_name>,string=<string>
+
+Since v5.1, QEMU allows some objects to generate fw_cfg-specific content,
+the content is then associated with a "file" item using the 'gen_id' option
+in the command line, using the following syntax:
+
+ -object <generator-type>,id=<generated_id>,[generator-specific-options] \
+ -fw_cfg [name=]<item_name>,gen_id=<generated_id>
+
+See QEMU man page for more documentation.
+
+Using item_name with plain ASCII characters only is recommended.
+
+Item names beginning with "opt/" are reserved for users. QEMU will
+never create entries with such names unless explicitly ordered by the
+user.
+
+To avoid clashes among different users, it is strongly recommended
+that you use names beginning with opt/RFQDN/, where RFQDN is a reverse
+fully qualified domain name you control. For instance, if SeaBIOS
+wanted to define additional names, the prefix "opt/org.seabios/" would
+be appropriate.
+
+For historical reasons, "opt/ovmf/" is reserved for OVMF firmware.
+
+Prefix "opt/org.qemu/" is reserved for QEMU itself.
+
+Use of names not beginning with "opt/" is potentially dangerous and
+entirely unsupported. QEMU will warn if you try.
+
+Use of names not beginning with "opt/" is tolerated with 'gen_id' (that
+is, the warning is suppressed), but you must know exactly what you're
+doing.
+
+All externally provided fw_cfg items are read-only to the guest.
+++ /dev/null
-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)!
-
-NOTE: As of QEMU v2.9, writes are reinstated, but only through the DMA
- interface (see below). Furthermore, writeability of any specific item is
- governed independently of Bit14 in the selector key value.
-
-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, but see the DMA interface)
-* 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
-DMA Address IOport: 0x514
-
-=== Arm Register Locations ===
-
-Selector Register address: Base + 8 (2 bytes)
-Data Register address: Base + 0 (8 bytes)
-DMA Address address: Base + 16 (8 bytes)
-
-== ACPI Interface ==
-
-The fw_cfg device is defined with ACPI ID "QEMU0002". Since we expect
-ACPI tables to be passed into the guest through the fw_cfg device itself,
-the guest-side firmware can not use ACPI to find fw_cfg. However, once the
-firmware is finished setting up ACPI tables and hands control over to the
-guest kernel, the latter can use the fw_cfg ACPI node for a more accurate
-inventory of in-use IOport or MMIO regions.
-
-== 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".
-
-If the DMA interface is available, then reading the DMA Address
-Register returns 0x51454d5520434647 ("QEMU CFG" in big-endian format).
-
-=== Revision / feature bitmap (Key 0x0001, FW_CFG_ID) ===
-
-A 32-bit little-endian unsigned int, this item is used to check for enabled
-features.
- - Bit 0: traditional interface. Always set.
- - Bit 1: DMA interface.
-
-=== 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, format and writeability.
-
-=== 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, generally RO, possibly RW through
- the DMA interface in QEMU v2.9+)
-0x4000 - 0x7fff Generic (0x0000 - 0x3fff, RW, ignored in QEMU v2.4+)
-0x8000 - 0xbfff Arch. Specific (0x0000 - 0x3fff, generally RO, possibly RW
- through the DMA interface in QEMU v2.9+)
-0xc000 - 0xffff Arch. Specific (0x0000 - 0x3fff, RW, ignored in v2.4+)
-
-In practice, the number of allowed firmware configuration items depends on the
-machine type/version.
-
-= Guest-side DMA Interface =
-
-If bit 1 of the feature bitmap is set, the DMA interface is present. This does
-not replace the existing fw_cfg interface, it is an add-on. This interface
-can be used through the 64-bit wide address register.
-
-The address register is in big-endian format. The value for the register is 0
-at startup and after an operation. A write to the least significant half (at
-offset 4) triggers an operation. This means that operations with 32-bit
-addresses can be triggered with just one write, whereas operations with
-64-bit addresses can be triggered with one 64-bit write or two 32-bit writes,
-starting with the most significant half (at offset 0).
-
-In this register, the physical address of a FWCfgDmaAccess structure in RAM
-should be written. This is the format of the FWCfgDmaAccess structure:
-
-typedef struct FWCfgDmaAccess {
- uint32_t control;
- uint32_t length;
- uint64_t address;
-} FWCfgDmaAccess;
-
-The fields of the structure are in big endian mode, and the field at the lowest
-address is the "control" field.
-
-The "control" field has the following bits:
- - Bit 0: Error
- - Bit 1: Read
- - Bit 2: Skip
- - Bit 3: Select. The upper 16 bits are the selected index.
- - Bit 4: Write
-
-When an operation is triggered, if the "control" field has bit 3 set, the
-upper 16 bits are interpreted as an index of a firmware configuration item.
-This has the same effect as writing the selector register.
-
-If the "control" field has bit 1 set, a read operation will be performed.
-"length" bytes for the current selector and offset will be copied into the
-physical RAM address specified by the "address" field.
-
-If the "control" field has bit 4 set (and not bit 1), a write operation will be
-performed. "length" bytes will be copied from the physical RAM address
-specified by the "address" field to the current selector and offset. QEMU
-prevents starting or finishing the write beyond the end of the item associated
-with the current selector (i.e., the item cannot be resized). Truncated writes
-are dropped entirely. Writes to read-only items are also rejected. All of these
-write errors set bit 0 (the error bit) in the "control" field.
-
-If the "control" field has bit 2 set (and neither bit 1 nor bit 4), a skip
-operation will be performed. The offset for the current selector will be
-advanced "length" bytes.
-
-To check the result, read the "control" field:
- error bit set -> something went wrong.
- all bits cleared -> transfer finished successfully.
- otherwise -> transfer still in progress (doesn't happen
- today due to implementation not being async,
- but may in the future).
-
-= Externally Provided Items =
-
-Since v2.4, "file" fw_cfg items (i.e., items with selector keys above
-FW_CFG_FILE_FIRST, and with a corresponding entry in the fw_cfg file
-directory structure) may be inserted via the QEMU command line, using
-the following syntax:
-
- -fw_cfg [name=]<item_name>,file=<path>
-
-Or
-
- -fw_cfg [name=]<item_name>,string=<string>
-
-Since v5.1, QEMU allows some objects to generate fw_cfg-specific content,
-the content is then associated with a "file" item using the 'gen_id' option
-in the command line, using the following syntax:
-
- -object <generator-type>,id=<generated_id>,[generator-specific-options] \
- -fw_cfg [name=]<item_name>,gen_id=<generated_id>
-
-See QEMU man page for more documentation.
-
-Using item_name with plain ASCII characters only is recommended.
-
-Item names beginning with "opt/" are reserved for users. QEMU will
-never create entries with such names unless explicitly ordered by the
-user.
-
-To avoid clashes among different users, it is strongly recommended
-that you use names beginning with opt/RFQDN/, where RFQDN is a reverse
-fully qualified domain name you control. For instance, if SeaBIOS
-wanted to define additional names, the prefix "opt/org.seabios/" would
-be appropriate.
-
-For historical reasons, "opt/ovmf/" is reserved for OVMF firmware.
-
-Prefix "opt/org.qemu/" is reserved for QEMU itself.
-
-Use of names not beginning with "opt/" is potentially dangerous and
-entirely unsupported. QEMU will warn if you try.
-
-Use of names not beginning with "opt/" is tolerated with 'gen_id' (that
-is, the warning is suppressed), but you must know exactly what you're
-doing.
-
-All externally provided fw_cfg items are read-only to the guest.
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