Re: [PATCH 02/18] fuse: Allow exporting BDSs via FUSE

From: Kevin Wolf <kwolf@redhat.com>
To: Markus Armbruster <armbru@redhat.com>
Cc: qemu-devel@nongnu.org, qemu-block@nongnu.org,
	Max Reitz <mreitz@redhat.com>
Subject: Re: [PATCH 02/18] fuse: Allow exporting BDSs via FUSE
Date: Fri, 20 Dec 2019 14:02:07 +0100	[thread overview]
Message-ID: <20191220130207.GH4019@dhcp-200-226.str.redhat.com> (raw)
In-Reply-To: <87lfr7go0z.fsf@dusky.pond.sub.org>

Am 20.12.2019 um 13:49 hat Markus Armbruster geschrieben:
> Kevin Wolf <kwolf@redhat.com> writes:
> 
> > Am 19.12.2019 um 15:38 hat Max Reitz geschrieben:
> >> fuse-export-add allows mounting block graph nodes via FUSE on some
> >> existing regular file.  That file should then appears like a raw disk
> >> image, and accesses to it result in accesses to the exported BDS.
> >> 
> >> Right now, we only set up the mount point and tear all mount points down
> >> in bdrv_close_all().  We do not implement any access functions, so
> >> accessing the mount point only results in errors.  This will be
> >> addressed by a followup patch.
> >> 
> >> The set of exported nodes is kept in a hash table so we can later add a
> >> fuse-export-remove that allows unmounting.
> >> 
> >> Signed-off-by: Max Reitz <mreitz@redhat.com>
> >
> >> diff --git a/qapi/block.json b/qapi/block.json
> >> index 145c268bb6..03f8d1b537 100644
> >> --- a/qapi/block.json
> >> +++ b/qapi/block.json
> >> @@ -317,6 +317,29 @@
> >>  ##
> >>  { 'command': 'nbd-server-stop' }
> >>  
> >> +##
> >> +# @fuse-export-add:
> >> +#
> >> +# Exports a block graph node on some (file) mountpoint as a raw image.
> >> +#
> >> +# @node-name: Node to be exported
> >> +#
> >> +# @mountpoint: Path on which to export the block device via FUSE.
> >> +#              This must point to an existing regular file.
> >> +#
> >> +# @writable: Whether clients should be able to write to the block
> >> +#            device via the FUSE export. (default: false)
> >> +#
> >> +# Since: 5.0
> >> +##
> >> +{ 'command': 'fuse-export-add',
> >> +  'data': {
> >> +      'node-name': 'str',
> >> +      'mountpoint': 'str',
> >> +      '*writable': 'bool'
> >> +  },
> >> +  'if': 'defined(CONFIG_FUSE)' }
> >
> > Can this use a BlockExport union from the start like I'm introducing in
> > the storage daemon series, together with a generic block-export-add?
> >
> > It also looks like node-name and writable should be part of the common
> > base of BlockExport. Unfortunately this would mean that I can't use the
> > same BlockExportNbd for the existing nbd-server-add command any more. I
> > guess I could somehow get a shared base type for both, though.
> >
> > Markus, any thoughts on these QAPI interfaces?
> 
> Context?  How far back should I read?

Basically just the hunk quoted above and the QAPI portion of the
following storage daemon patch:

    [RFC PATCH 08/18] qemu-storage-daemon: Add --export option

Maybe the cover letter, too, if you need a more high-level introduction.

Kevin