On Thu, Oct 11, 2012 at 10:33:31AM +1030, Rusty Russell wrote: > Paolo Bonzini <pbonzini@xxxxxxxxxx> writes: > > Il 09/10/2012 06:59, Rusty Russell ha scritto: > >> Paolo Bonzini <pbonzini@xxxxxxxxxx> writes: > >>> Il 05/10/2012 07:43, Rusty Russell ha scritto: > >>>> That's good. But virtio_blk's scsi command is insoluble AFAICT. As I > >>>> said to Anthony, the best rules are "always" and "never", so I'd really > >>>> rather not have to grandfather that in. > >>> > >>> It is, but we can add a rule that if the (transport) flag > >>> VIRTIO_RING_F_ANY_HEADER_SG is set, the cdb field is always 32 bytes in > >>> virtio-blk. > >> > >> Could we do that? It's the cmd length I'm concerned about; is it always > >> 32 in practice for some reason? > > > > It is always 32 or less except in very obscure cases that are pretty > > much confined to iSCSI. We don't care about the obscure cases, and the > > extra bytes don't hurt. > > > > BTW, 32 is the default cdb_size used by virtio-scsi. > > > >> Currently qemu does: > >> > >> struct sg_io_hdr hdr; > >> memset(&hdr, 0, sizeof(struct sg_io_hdr)); > >> hdr.interface_id = 'S'; > >> hdr.cmd_len = req->elem.out_sg[1].iov_len; > >> hdr.cmdp = req->elem.out_sg[1].iov_base; > >> hdr.dxfer_len = 0; > >> > >> If it's a command which expects more output data, there's no way to > >> guess where the boundary is between that command and the data. > > > > Yep, so I understood the problem right. > > OK. Well, Anthony wants qemu to be robust in this regard, so I am > tempted to rework all the qemu drivers to handle arbitrary layouts. > They could use a good audit anyway. I agree here. Still trying to understand whether we can agree to use a feature bit for this, or not. > This would become a glaring exception, but I'm tempted to fix it to 32 > bytes at the same time as we get the new pci layout (ie. for the virtio > 1.0 spec). But this isn't a virtio-pci only issue, is it? qemu has s390 bus with same limmitation. How can we tie it to pci layout? Maybe what you mean is to use a transport feature for this and tie *that* to new layout in case of pci? > The Linux driver would carefully be backwards compatible, of > course, and the spec would document why. > > Cheers, > Rusty. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
