On Thu, Jul 04, 2013 at 10:49:42AM +0300, Michael S. Tsirkin wrote: > 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. Rusty, could you please tell me which feature bit do you prefer for ANY_LAYOUT? It would be sad to miss another release of both qemu and kernel with this obvious optimization for the only reason we can't settle on a bit to use to signal it. > -- > MST _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
