Re: [PATCH v2 11/13] block/export: convert vhost-user-blk server to block export API

[Markus Armbruster]

Stefan Hajnoczi <stefanha@redhat.com> writes:

> On Wed, Sep 30, 2020 at 07:28:58AM +0200, Markus Armbruster wrote:
>> Stefan Hajnoczi <stefanha@redhat.com> writes:
>> 
>> > Use the new QAPI block exports API instead of defining our own QOM
>> > objects.
>> >
>> > This is a large change because the lifecycle of VuBlockDev needs to
>> > follow BlockExportDriver. QOM properties are replaced by QAPI options
>> > objects.
>> >
>> > VuBlockDev is renamed VuBlkExport and contains a BlockExport field.
>> > Several fields can be dropped since BlockExport already has equivalents.
>> >
>> > The file names and meson build integration will be adjusted in a future
>> > patch. libvhost-user should probably be built as a static library that
>> > is linked into QEMU instead of as a .c file that results in duplicate
>> > compilation.
>> >
>> > The new command-line syntax is:
>> >
>> >   $ qemu-storage-daemon \
>> >       --blockdev file,node-name=drive0,filename=test.img \
>> >       --export 
>> > vhost-user-blk,node-name=drive0,id=export0,unix-socket=/tmp/vhost-user-blk.sock
>> >
>> > Note that unix-socket is optional because we may wish to accept chardevs
>> > too in the future.
>> >
>> > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
>> > ---
>> > v2:
>> >  * Replace str unix-socket with SocketAddress addr to match NBD and
>> >    support file descriptor passing
>> >  * Make addr mandatory [Markus]
>> >  * Update vhost-user-blk-test.c to use --export syntax
>> > ---
>> >  qapi/block-export.json               |  21 +-
>> >  block/export/vhost-user-blk-server.h |  23 +-
>> >  block/export/export.c                |   8 +-
>> >  block/export/vhost-user-blk-server.c | 452 +++++++--------------------
>> >  tests/qtest/vhost-user-blk-test.c    |   2 +-
>> >  util/vhost-user-server.c             |  10 +-
>> >  block/export/meson.build             |   1 +
>> >  block/meson.build                    |   1 -
>> >  8 files changed, 158 insertions(+), 360 deletions(-)
>> >
>> > diff --git a/qapi/block-export.json b/qapi/block-export.json
>> > index ace0d66e17..2e44625bb1 100644
>> > --- a/qapi/block-export.json
>> > +++ b/qapi/block-export.json
>> > @@ -84,6 +84,21 @@
>> >    'data': { '*name': 'str', '*description': 'str',
>> >              '*bitmap': 'str' } }
>> >  
>> > +##
>> > +# @BlockExportOptionsVhostUserBlk:
>> > +#
>> > +# A vhost-user-blk block export.
>> > +#
>> > +# @addr: The vhost-user socket on which to listen. Both 'unix' and 'fd'
>> > +#        SocketAddress types are supported. Passed fds must be UNIX domain
>> > +#        sockets.
>> 
>> "addr.type must be 'unix' or 'fd'" is not visible in introspection.
>> Awkward.  Practical problem only if other addresses ever become
>> available here.  Is that possible?
>
> addr.type=fd itself has the same problem, because it is a file
> descriptor without type information. Therefore the QMP client cannot
> introspect which types of file descriptors can be passed.

Yes, but if introspection could tell us which which values of addr.type
are valid, then a client should figure out the address families, as
follows.  Any valid value other than 'fd' corresponds to an address
family.  The set of values valid for addr.type therefore corresponds to
a set of address families.  The address families in that set are all
valid with 'fd', aren't they?

> Two ideas:
>
> 1. Introduce per-address family fd types (SocketAddrFdTcpInet,
>    SocketAddrFdTcpInet6, SocketAddrFdUnix, etc) to express specific
>    address families in the QAPI schema.
>
>    Then use per-command unions to express the address families supported
>    by specific commands. For example,
>    BlockExportOptionsVhostUserBlkSocketAddr would only allow
>    SocketAddrUnix and SocketAddrFdUnix. That way new address families
>    can be supported in the future and introspection reports.

Awkward.  These types would have to differ structurally, or else they
are indistinguishable in introspection.

> 2. Use a side-channel (query-features, I think we discussed something
>    like this a while back) to report features that cannot be
>    introspected.

We implemented this in the form of QAPI feature flags, visible in
introspection.  You could do something like

  'addr': { 'type': 'SocketAddress',
            'features': [ 'unix' ] }

> I think the added complexity for achieving full introspection is not
> worth it. It becomes harder to define new QAPI commands, increases the
> chance of errors, and is more tedious to program for clients/servers.

Hence my question: is it possible that address families other than unix
become available here?

When that happens, we have an introspection problem of the sort we
common solve with a feature flag.

> Accepting any SocketAddr seems reasonable to me since vhost-user
> requires an address family that has file descriptor passing. Very few
> address families support this feature and we don't expect to add new
> ones often.

Your answer appears to be "yes in theory, quite unlikely in practice".
Correct?