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Re: [Qemu-block] [Qemu-devel] [PATCH 2/2] qapi: Allow blockdev-add for N

From: Daniel P. Berrange
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH 2/2] qapi: Allow blockdev-add for NBD
Date: Wed, 3 Feb 2016 17:06:09 +0000
User-agent: Mutt/1.5.24 (2015-08-30)

On Wed, Feb 03, 2016 at 05:33:16PM +0100, Max Reitz wrote:
> We have to introduce a new object (BlockdevOptionsNbd) for several
> reasons:
> - Neither of InetSocketAddress nor UnixSocketAddress alone is
>   sufficient, because both are supported
> - We cannot use SocketAddress because NBD does not support an fd,
>   and because it is not a flat union which BlockdevOptionsNbd is

With my patch series that converts NBD to use QIOChannel, all the
entry points for client & server *do* take a SocketAddress struct
to provide address info. So internally the code does in fact allow
use of an FD, if there were a way to specify it a the QAPI level...

eg see


> - We cannot use a flat union of InetSocketAddress and
>   UnixSocketAddress because we would need some kind of discriminator
>   which we do not have; we could inline the UnixSocketAddress as a
>   string and then make it an 'alternate' type instead of a union, but
>   this will not work either, because:
> - InetSocketAddress itself is not suitable for NBD because the port is
>   not optional (which it is for NBD) and because it offers more options
>   (like choosing between ipv4 and ipv6) which NBD does not support.

The *should* support ipv4 and ipv6 options for NBD. We should also make
the port optional in the SocketAddress struct - I tried to do that previously
but my patch was flawed, but we should revisit this.

So IMHO all the things you list above are reasons *for* using SocketAddress
and not re-inventing it poorly with explicit host + port fields.

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