On Wed, Jun 19, 2013 at 02:16:06PM +0930, Rusty Russell wrote: > "Michael S. Tsirkin" <mst@xxxxxxxxxx> writes: > > On Thu, Jun 13, 2013 at 11:10:47AM +0300, Michael S. Tsirkin wrote: > >> On Thu, Jun 13, 2013 at 11:02:59AM +0300, Michael S. Tsirkin wrote: > >> > On Thu, Jun 13, 2013 at 02:12:22PM +0930, Rusty Russell wrote: > >> > > "Michael S. Tsirkin" <mst@xxxxxxxxxx> writes: > >> > > > On Thu, Mar 14, 2013 at 04:15:28PM +0100, Paolo Bonzini wrote: > >> > > >> Il 14/03/2013 12:10, Michael S. Tsirkin ha scritto: > >> > > >> > Add field for guest to specify command size for virtio-blk. > >> > > >> > > >> > > >> > Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx> > >> > > > > >> > > > OK, but Rusty usually tweaks wording anyway. > >> > > > Rusty want to apply and make changes Paolo suggested yourself > >> > > > or want me to? > >> > > > >> > > (MST drew my attention back to this) > >> > > > >> > > Please do. And please add a note about this feature: that without it, > >> > > the descriptor layout must be on the right boundaries for historical > >> > > reasons. > >> > > >> > It's there already, isn't it: > >> > > >> > If VIRTIO_BLK_F_SCSI_CMD_SIZE is not negotiated, > >> > This field must reside in a single, separate read-only buffer; > >> > the command length > >> > can be derived from the length of this buffer. > >> > > >> > >> Wait, I think I got it: you actually want to rename this > >> VIRTIO_BLK_F_ANY_SG and have it affect all requests? > > > > Sorry about being dense - could you please clarify? > > OK, here's the new section I've written (but not committed!) on Message > Framing: > > 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, such VIRTIO_NET_F_ANY_LAYOUT or > VIRTIO_BLK_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 by dividing a > network packet into 1500 single-byte descriptors! > === > > Notes: > 1) The restrictions are still in place by default. > 2) We introduce VIRTIO_NET_F_ANY_LAYOUT and VIRTIO_BLK_F_ANY_LAYOUT > specifically for net and block (note the new names). > 3) I note the special case of stupid descriptors. > 4) Enjoy our humble pie, don't hide it in a footnote! > > If that seems OK, I'll modify the net and block specs appropriately. > > Cheers, > Rusty. Acked-by: Michael S. Tsirkin <mst@xxxxxxxxxx> an additional suggestion: limit max s/g to vq size (it's in theory possible to go higher with indirect descriptors, but we need some limit, and queue size gives us a nice way to say "this is a max descriptor size"). -- MST _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
