Anthony Liguori <anthony@xxxxxxxxxxxxx> writes: >> lguest fix is pending in my queue. lkvm and qemu are broken; lkvm isn't >> ever going to be merged, so I'm not sure what its status is? But I'm >> determined to fix qemu, and hence my torture patch to make sure this >> doesn't creep in again. > > There are even more implementations out there and I'd wager they all > rely on framing. Worse, both virtio_blk (for scsi commands) and virtio_scsi explicitly and inescapably rely on framing. The spec conflicts clearly with itself. Such layering violations are always a mistake, but I can't blame anyone else for my lack of attention :( Here's the spec change: commit 7e74459bb966ccbaad9e4bf361d1178b7f400b79 Author: Rusty Russell <rusty@xxxxxxxxxxxxxxx> Date: Thu Oct 4 17:11:27 2012 +0930 No longer assume framing is independent of messages. *sniff* Signed-off-by: Rusty Russell <rusty@xxxxxxxxxxxxxxx> --- virtio-spec.txt 2012-10-04 17:13:04.988259234 +0930 +++ virtio-spec.txt.new 2012-10-04 17:12:54.624258969 +0930 @@ -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 _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
