On Fri, 13 Apr 2012 10:48:18 -0500, Anthony Liguori <aliguori@xxxxxxxxxx> wrote: > On 04/13/2012 09:50 AM, Stefan Hajnoczi wrote: > > The virtio specification says: > > > > "The descriptors used for a buffer should not effect the semantics > > of the message, > > except for the total length of the buffer" > > > > and > > > > "In particular, no implementation should use the descriptor boundaries > > to determine the size of any header in a request" > > This was the noble intention but all of the implementations actually rely on > boundary sizes. > > Both QEMU and lguest rely on boundary sizes. We've removed some of it in > virtio-net in QEMU but it still looks like it's there for virtio-blk. I will fix lguest. It's a poor excuse to say that lguest doens't have an ABI, so it can take these shortcuts; it's also the exemplar. > kvm tool also makes this assumption. > > > Why should descriptor layout not be specified? > > > > It seems that implementing arbitrary descriptor layout support (e.g. > > 1-byte descriptors) requires more code and makes input validation > > harder. > > > > Why bother with the flexibility of unspecified descriptor layouts? As > > long as the layout is specified clearly it makes everyone's lives > > easier to use a strict descriptor layout. > > I hate to just change the spec here but I don't see a better option. For example, in the net code, we could often pack the virtio_net header into the skb and save a descriptor. We didn't do this because it would break qemu. Added to TODO list: a new DEBUG config option which breaks up virtio descriptors. Thanks, Rusty. _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
