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Subject: Re: [PATCH 2/2] nbd: add support for nbd as root device
To:     roman.stratiienko@globallogic.com, linux-kernel@vger.kernel.org,
        josef@toxicpanda.com, nbd@other.debian.org,
        A.Bulyshchenko@globallogic.com, linux-block@vger.kernel.org,
        axboe@kernel.dkn.org
References: <20190612163144.18486-1-roman.stratiienko@globallogic.com>
 <20190612163144.18486-2-roman.stratiienko@globallogic.com>
From:   Eric Blake <eblake@redhat.com>
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Organization: Red Hat, Inc.
Message-ID: <3a15a92f-16d6-cca3-8842-b0866579a95a@redhat.com>
Date:   Thu, 13 Jun 2019 07:56:38 -0500
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From: Eric Blake <eblake@redhat.com>
To: roman.stratiienko@globallogic.com, linux-kernel@vger.kernel.org,
 josef@toxicpanda.com, nbd@other.debian.org, A.Bulyshchenko@globallogic.com,
 linux-block@vger.kernel.org, axboe@kernel.dkn.org
Message-ID: <3a15a92f-16d6-cca3-8842-b0866579a95a@redhat.com>
Subject: Re: [PATCH 2/2] nbd: add support for nbd as root device
References: <20190612163144.18486-1-roman.stratiienko@globallogic.com>
 <20190612163144.18486-2-roman.stratiienko@globallogic.com>
In-Reply-To: <20190612163144.18486-2-roman.stratiienko@globallogic.com>

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On 6/12/19 11:31 AM, roman.stratiienko@globallogic.com wrote:
> From: Roman Stratiienko <roman.stratiienko@globallogic.com>
>=20
> Adding support to nbd to use it as a root device. This code essentially=

> provides a minimal nbd-client implementation within the kernel. It open=
s
> a socket and makes the negotiation with the server. Afterwards it passe=
s
> the socket to the normal nbd-code to handle the connection.
>=20
> The arguments for the server are passed via kernel command line.
> The kernel command line has the format
> 'nbdroot=3D[<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.
>=20
> Based on those arguments, the connection to the server is established
> and is connected to the nbd0 device. The rootdevice therefore is
> root=3D/dev/nbd0.
>=20
> Patch was initialy posted by Markus Pargmann <mpa@pengutronix.de>
> and can be found at https://lore.kernel.org/patchwork/patch/532556/
>=20
> Change-Id: I78f7313918bf31b9dc01a74a42f0f068bede312c
> Signed-off-by: Roman Stratiienko <roman.stratiienko@globallogic.com>
> Reviewed-by: Aleksandr Bulyshchenko <A.Bulyshchenko@globallogic.com>

> +static int nbd_connection_negotiate(struct socket *sock, char *export_=
name,
> +				    size_t *rsize, u16 *nflags)
> +{

> +
> +	ret =3D sock_xmit_buf(sock, 0, &flags, sizeof(flags));
> +	if (ret < 0)
> +		return ret;
> +
> +	*nflags =3D ntohs(flags);
> +
> +	client_flags =3D 0;

Why not reply with NBD_FLAG_C_FIXED_NEWSTYLE? Which in turn would be
necessary...

> +
> +	ret =3D sock_xmit_buf(sock, 1, &client_flags, sizeof(client_flags));
> +	if (ret < 0)
> +		return ret;
> +
> +	magic =3D cpu_to_be64(nbd_opts_magic);
> +	ret =3D sock_xmit_buf(sock, 1, &magic, sizeof(magic));
> +	if (ret < 0)
> +		return ret;
> +
> +	opt =3D 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).

=2E..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.

--=20
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org


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