Re: [PATCH] virtio-spec: add field for scsi command size

["Michael S. Tsirkin" <mst@redhat.com>]

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@redhat.com> 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