diff options
Diffstat (limited to 'docs/specs/fw_cfg.txt')
| -rw-r--r-- | docs/specs/fw_cfg.txt | 32 |
1 files changed, 25 insertions, 7 deletions
diff --git a/docs/specs/fw_cfg.txt b/docs/specs/fw_cfg.txt index 7a5f8c7824..a19e2adbe1 100644 --- a/docs/specs/fw_cfg.txt +++ b/docs/specs/fw_cfg.txt @@ -33,6 +33,10 @@ 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 @@ -43,7 +47,7 @@ value between 0x8000-0xffff. == Data Register == -* Read/Write (writes ignored as of QEMU v2.4) +* 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 @@ -134,8 +138,8 @@ struct FWCfgFile { /* an individual file entry, 64 bytes total */ === 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. +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 === @@ -144,9 +148,11 @@ items, and up to 0x4000 architecturally specific ones. Selector Reg. Range Usage --------------- ----------- -0x0000 - 0x3fff Generic (0x0000 - 0x3fff, RO) +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, RO) +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 is given @@ -182,6 +188,7 @@ The "control" field has the following bits: - 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. @@ -191,8 +198,17 @@ 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 2 set (and not bit 1), a skip operation will be -performed. The offset for the current selector will be advanced "length" bytes. +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. @@ -234,3 +250,5 @@ 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. + +All externally provided fw_cfg items are read-only to the guest. |