Paolo Bonzini <pbonzini@xxxxxxxxxx> writes: > Il 01/07/2013 01:47, Rusty Russell ha scritto: >> > > >> > > Mainly because I'm not sure that *all* devices are now safe. Are they? >> > >> > virtio-scsi's implementation in QEMU is not safe (been delaying that for >> > too long, sorry), but the spec is safe. >> >> Then if we added a transport feature, we couldn't use it :( > > Transport feature bits are still negotiated per device though. > virtio-scsi devices in QEMU would not negotiate that feature. That's a good point; I tend to think of them as tied to the transport but there's nothing specifying that, nor any implementation requiring it. OK, so VIRTIO_F_ANY_LAYOUT it is? Cheers, Rusty. 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). Regrettably, initial driver implementations used simple layouts and devices came to rely on it, despite this specification wording. It is thus recommended that drivers be conservative in their assumptions, unless specific device features indicate that general layout is permitted using VIRTIO_F_ANY_LAYOUT. 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 dividing a network packet into 1500 single-byte descriptors! _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
