On Tue, Jul 02, 2013 at 03:34:09PM +0930, Rusty Russell wrote: > 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! That's fine with me too. So which bit are we using for this? I'd like to rebase to latest bits and merge the optimization for 3.11. -- MST _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
