Re: [Qemu-devel] [PATCH v2 2/2] qapi: allow blockdev-add for NFS

[Kevin Wolf <kwolf@redhat.com>]

Am 26.10.2016 um 09:23 hat Markus Armbruster geschrieben:
> A few drive-by comments...
> 
> Eric Blake <eblake@redhat.com> writes:
> 
> > On 10/24/2016 02:27 PM, Ashijeet Acharya wrote:
> >> Introduce new object 'BlockdevOptionsNFS' in qapi/block-core.json to
> >> support blockdev-add for NFS network protocol driver. Also make a new
> >> struct NFSServer to support tcp connection.
> >> 
> >> Signed-off-by: Ashijeet Acharya <ashijeetacharya@gmail.com>
> >> ---
> >>  qapi/block-core.json | 56 ++++++++++++++++++++++++++++++++++++++++++++++++----
> >>  1 file changed, 52 insertions(+), 4 deletions(-)
> >> 
> >> diff --git a/qapi/block-core.json b/qapi/block-core.json
> >> index 9d797b8..3ab028d 100644
> >> --- a/qapi/block-core.json
> >> +++ b/qapi/block-core.json
> >> @@ -1714,9 +1714,9 @@
> >>  { 'enum': 'BlockdevDriver',
> >>    'data': [ 'archipelago', 'blkdebug', 'blkverify', 'bochs', 'cloop',
> >>              'dmg', 'file', 'ftp', 'ftps', 'gluster', 'host_cdrom',
> >> -            'host_device', 'http', 'https', 'luks', 'null-aio', 'null-co',
> >> -            'parallels', 'qcow', 'qcow2', 'qed', 'quorum', 'raw',
> >> -	    'replication', 'tftp', 'vdi', 'vhdx', 'vmdk', 'vpc', 'vvfat' ] }
> >> +            'host_device', 'http', 'https', 'luks', 'nfs', 'null-aio',
> >> +            'null-co', 'parallels', 'qcow', 'qcow2', 'qed', 'quorum', 'raw',
> >> +            'replication', 'tftp', 'vdi', 'vhdx', 'vmdk', 'vpc', 'vvfat' ] }
> >
> > Missing a comment that 'nfs' is since 2.8.
> >
> >>  ##
> >> +# @NFSServer
> >> +#
> >> +# Captures the address of the socket
> >> +#
> >> +# @type:        transport type used for NFS (only TCP supported)
> >> +#
> >> +# @host:        host part of the address
> >> +#
> >> +# Since 2.8
> >> +##
> >> +{ 'struct': 'NFSServer',
> >> +  'data': { 'type': 'str',
> >
> > Please make this an enum, instead of an open-coded string. It's okay if
> > the enum only has one value 'tcp' for now; but using an enum will make
> > it introspectable if we later add a second transport, unlike what we get
> > with an open-coded string.
> 
> Yes.  When a JSON string has a compile-time fixed set of values, 'str'
> is generally wrong.
> 
> > Must 'type' be mandatory if it must always be 'tcp'?
> >
> >> +            'host': 'str' } }
> >> +
> >> +##
> >> +# @BlockdevOptionsNfs
> >> +#
> >> +# Driver specific block device option for NFS
> >> +#
> >> +# @server:        host address
> >> +#
> >> +# @path:          path of the image on the host
> >> +#
> >> +# @uid:           #optional UID value to use when talking to the server
> >> +#
> >> +# @gid:           #optional GID value to use when talking to the server
> >
> > Do we want to allow string names in addition to numeric uid/gid values?
> > I'm not sure if NFS has name-based id mapping, but it's food for thought
> > on whether we need to use an alternate type here (alternate between
> > integer id and string name), or leave this as is.
> 
> As far as I know, NFS4 supports user and group names.  On Linux, see
> rpc.idmapd(8).
> 
> How the name support affects C code I can't say.  If it's transparent,
> i.e. you simply use local UID/GID, and the mapping happens below the
> hood, then we'd have to translate string values to local UID/GID by the
> usual means.  Looks like a minor convenience feature on first glance.
> However, QMP is a *network* protocol.  A remote client can't easily do
> this translation.
> 
> Consider a GUI like virt-manager: I guess we'd rather support user and
> group names there.  But if we do, and QMP doesn't, either virt-manager
> or libvirt need to map to numeric IDs.  Easy enough when running on the
> host, probably impractical when not.
> 
> If we permit string values, are @uid and @gid still appropriate names?
> The user name is not the user ID, it just maps to it.
>
> >> +#
> >> +# @tcp-syncnt:    #optional number of SYNs during the session establishment
> >
> > Would tcp-syn-count be any more legible?
> 
> We generally write out things in long hand in the QAPI schema.
> 
> >                                           What is the default when omitted?
> 
> Whenever you write #optional, you must explain the default.  When
> the default is a fixed value, specify it.  When the system picks a
> default, state that, and think hard about what you need to specify on
> how the system picks.
> 
> >> +#
> >> +# @readahead:     #optional set the readahead size in bytes
> 
> @read-ahead
> 
> > What's the default when omitted?
> >
> >> +#
> >> +# @pagecache:     #optional set the pagecache size in bytes
> 
> @page-cache
> 
> > Default?
> >
> >> +#
> >> +# @debug:         #optional set the NFS debug level (max 2)
> >
> > Presumably default 0?
> 
> @BlockdevOptionsGluster calls this @debug-level.

Are all of these renames really worth having to support two option
names for each option in the driver? I'm not sure if I'm convinced of
this.

Kevin