@@ -880,19 +880,19 @@
Message Framing
-The descriptors used for a buffer should not effect the semantics
-of the message, except for the total length of the buffer. For
-example, a network buffer consists of a 10 byte header followed
-by the network packet. Whether this is presented in the ring
-descriptor chain as (say) a 10 byte buffer and a 1514 byte
-buffer, or a single 1524 byte buffer, or even three buffers,
-should have no effect.
+Unless stated otherwise, it is expected that headers within a
+message are contained within their own descriptors. For example,
+a network buffer consists of a 10 or 12 byte header followed by
+the network packet. An implementation should expect that this
+header will be within the first descriptor, and that the
+remainder of the data will begin on the second descriptor.
-In particular, no implementation should use the descriptor
-boundaries to determine the size of any header in a request.[footnote:
-The current qemu device implementations mistakenly insist that
-the first descriptor cover the header in these cases exactly, so
-a cautious driver should arrange it so.
+[footnote:
+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.
]
Device Improvements