On 6/12/19 11:31 AM, roman.stratiienko@xxxxxxxxxxxxxxx wrote: > From: Roman Stratiienko <roman.stratiienko@xxxxxxxxxxxxxxx> > > Adding support to nbd to use it as a root device. This code essentially > provides a minimal nbd-client implementation within the kernel. It opens > a socket and makes the negotiation with the server. Afterwards it passes > the socket to the normal nbd-code to handle the connection. > > The arguments for the server are passed via kernel command line. > The kernel command line has the format > 'nbdroot=[<SERVER_IP>:]<SERVER_PORT>/<EXPORT_NAME>'. > SERVER_IP is optional. If it is not available it will use the > root_server_addr transmitted through DHCP. > > Based on those arguments, the connection to the server is established > and is connected to the nbd0 device. The rootdevice therefore is > root=/dev/nbd0. > > Patch was initialy posted by Markus Pargmann <mpa@xxxxxxxxxxxxxx> > and can be found at https://lore.kernel.org/patchwork/patch/532556/ > > Change-Id: I78f7313918bf31b9dc01a74a42f0f068bede312c > Signed-off-by: Roman Stratiienko <roman.stratiienko@xxxxxxxxxxxxxxx> > Reviewed-by: Aleksandr Bulyshchenko <A.Bulyshchenko@xxxxxxxxxxxxxxx> > +static int nbd_connection_negotiate(struct socket *sock, char *export_name, > + size_t *rsize, u16 *nflags) > +{ > + > + ret = sock_xmit_buf(sock, 0, &flags, sizeof(flags)); > + if (ret < 0) > + return ret; > + > + *nflags = ntohs(flags); > + > + client_flags = 0; Why not reply with NBD_FLAG_C_FIXED_NEWSTYLE? Which in turn would be necessary... > + > + ret = sock_xmit_buf(sock, 1, &client_flags, sizeof(client_flags)); > + if (ret < 0) > + return ret; > + > + magic = cpu_to_be64(nbd_opts_magic); > + ret = sock_xmit_buf(sock, 1, &magic, sizeof(magic)); > + if (ret < 0) > + return ret; > + > + opt = htonl(NBD_OPT_EXPORT_NAME); Why not use NBD_OPT_GO? That's a better interface (better error recovery, and some servers can even use it to advertise preferred block sizing - particularly if a server insists on a 4k minimum block instead of 512). ...using NBD_OPT_GO would require checking that the server advertised FIXED_NEWSTYLE as well as replying that we intend to rely on it. Otherwise the idea looks interesting to me. -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org
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