diff options
| -rw-r--r-- | virtio-v1.0-wd01-part1-specification.txt | 65 |
1 files changed, 39 insertions, 26 deletions
diff --git a/virtio-v1.0-wd01-part1-specification.txt b/virtio-v1.0-wd01-part1-specification.txt index b4298bb..9af2072 100644 --- a/virtio-v1.0-wd01-part1-specification.txt +++ b/virtio-v1.0-wd01-part1-specification.txt @@ -269,25 +269,34 @@ is not a significant issue. 2.1.4.2. Message Framing ----------------------- -The original intent of the specification was that message framing (the -particular layout of descriptors) be independent of the contents of -the buffers. For example, a network transmit buffer consists of a 12 -byte header followed by the network packet. This could be most simply -placed in the descriptor table as a 12 byte output descriptor followed -by a 1514 byte output descriptor, but it could also consist of a -single 1526 byte output descriptor in the case where the header and -packet are adjacent, or even three or more descriptors (possibly with -loss of efficiency in that case). +The message framing (the particular layout of descriptors) is +independent of the contents of the buffers. For example, a network +transmit buffer consists of a 12 byte header followed by the network +packet. This could be most simply placed in the descriptor table as a +12 byte output descriptor followed by a 1514 byte output descriptor, +but it could also consist of a single 1526 byte output descriptor in +the case where the header and packet are adjacent, or even three or +more descriptors (possibly with loss of efficiency in that case). + +Note that, some implementations may have large-but-reasonable +restrictions on total descriptor size (such as based on IOV_MAX in the +host OS). This has not been a problem in practice: little sympathy +will be given to drivers which create unreasonably-sized descriptors +such as by dividing a network packet into 1500 single-byte +descriptors! + +2.1.4.2.1. Legacy Interface: Message Framing +----------------------- Regrettably, initial driver implementations used simple layouts, and -devices came to rely on it, despite this specification wording[10]. It -is thus recommended that drivers be conservative in their assumptions, -unless the VIRTIO_F_ANY_LAYOUT feature is accepted. In addition, some -implementations may have large-but-reasonable restrictions on total -descriptor size (such as based on IOV_MAX in the host OS). This has -not been a problem in practice: little sympathy will be given to -drivers which create unreasonably-sized descriptors such as by -dividing a network packet into 1500 single-byte descriptors! +devices came to rely on it, despite this specification wording. In +addition, the specification for virtio_blk SCSI commands required +intuiting field lengths from frame boundaries (see "2.4.2.5.1 Legacy +Interface: Device Operation") + +It is thus recommended that when using legacy interfaces, transitional +drivers be conservative in their assumptions, unless the +VIRTIO_F_ANY_LAYOUT feature is accepted. 2.1.4.3. The Virtqueue Descriptor Table -------------------------------------- @@ -1730,6 +1739,9 @@ error or VIRTIO_BLK_S_UNSUPP for a request unsupported by host: #define VIRTIO_BLK_S_IOERR 1 #define VIRTIO_BLK_S_UNSUPP 2 +2.4.2.5.1 Legacy Interface: Device Operation +------------------------ + Historically, devices assumed that the fields type, ioprio and sector reside in a single, separate read-only buffer; the fields errors, data_len, sense_len and residual reside in a single, @@ -2584,9 +2596,6 @@ Currently there are five device-independent feature bits defined: using a timer if the device interrupts it when all the packets are transmitted. - VIRTIO_F_ANY_LAYOUT (27) This feature indicates that the device accepts arbitrary - descriptor layouts, as described in Section "2.1.4.2. Message Framing". - VIRTIO_F_RING_INDIRECT_DESC (28) Negotiating this feature indicates that the driver can use descriptors with the VRING_DESC_F_INDIRECT flag set, as described in "2.1.4.3.1. Indirect Descriptors". @@ -2610,6 +2619,16 @@ In addition, bit 30 is used by qemu's implementation to check for experimental early versions of virtio which did not perform correct feature negotiation, and should not be used. +2.5.1 Legacy Interface: Reserved Feature Bits +-------------------------------------------- + +Legacy or transitional devices may offer the following: + +VIRTIO_F_ANY_LAYOUT (27) This feature indicates that the device + accepts arbitrary descriptor layouts, as described in Section + "2.1.4.2.1. Legacy Interface: Message Framing". + + 2.6. virtio_ring.h ================= @@ -2884,12 +2903,6 @@ of this expected condition is necessary. [9] https://lists.linux-foundation.org/mailman/listinfo/virtualization -[10] It was previously asserted that framing should be independent of message -contents, yet invariably drivers layed out messages in reliable ways and -devices assumed it. -In addition, the specifications for virtio_blk and virtio_scsi require -intuiting field lengths from frame boundaries. - [11] Even if it does mean documenting design or implementation mistakes! |