Re: [Qemu-devel] [PATCH 06/11] chardev: add file chardev support to chardev-add (qmp)

[Eric Blake]

On 01/07/2013 06:55 AM, Gerd Hoffmann wrote:
> Add support for file chardevs.  Output file is mandatory,
> input file is optional.  Both file names and file descriptor
> passing is supported.
> 
> Signed-off-by: Gerd Hoffmann <address@hidden>
> ---
>  qapi-schema.json |    9 +++++-
>  qemu-char.c      |   77 
> +++++++++++++++++++++++++++++++++++++++++++++++++++++-
>  2 files changed, 84 insertions(+), 2 deletions(-)
> 
> diff --git a/qapi-schema.json b/qapi-schema.json
> index e3f0d44..8904d36 100644
> --- a/qapi-schema.json
> +++ b/qapi-schema.json
> @@ -3030,9 +3030,16 @@
>  #
>  # Since: 1.4
>  ##
> +{ 'union': 'ChardevFileSource', 'data': { 'path' : 'str',
> +                                          'fd'   : 'str' } }
> +
> +{ 'type': 'ChardevFile', 'data': { '*in' : 'ChardevFileSource',
> +                                   'out' : 'ChardevFileSource' } }
> +
>  { 'type': 'ChardevDummy', 'data': { } }
>  
> -{ 'union': 'ChardevBackend', 'data': { 'null' : 'ChardevDummy' } }
> +{ 'union': 'ChardevBackend', 'data': { 'file' : 'ChardevFile',
> +                                       'null' : 'ChardevDummy' } }
>  
>  { 'command': 'chardev-add', 'data': {'id'      : 'str',
>                                       'backend' : 'ChardevBackend' } }

An example in qmp-commands.hx would be helpful; am I correct that an
example would be:

-> { "execute" : "chardev-add",
     "arguments" : { "id" : "foo",
                     "backend" : { "type" : "file", "data" : {
         "in"  : { "type" : "fd", "data" : "namedfd" },
         "out" : { "type" : "path", "data" : "/path/to/file" } } } } }
<- { "return": {} }

where namedfd was previously given via 'getfd'?

Do we need the complexity of supporting fd passing explicitly?  'getfd'
is less than ideal compared to 'add-fd', and for 'add-fd', we would pass
via "path":"/dev/fdset/nnn".  That is, why do we need to bend over
backwards to support an alternate syntax for fd passing in a new
command, when we can already use existing commands to get fd passing for
free?

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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