Re: [Qemu-devel] [PATCH v7 2/6] qapi: Introduce add-fd, remove-fd, query-fdsets

[Corey Bryant]

On 08/07/2012 12:49 PM, Eric Blake wrote:
> On 08/07/2012 09:58 AM, Corey Bryant wrote:
> > This patch adds support that enables passing of file descriptors
> > to the QEMU monitor where they will be stored in specified file
> > descriptor sets.
> >
> > A file descriptor set can be used by a client like libvirt to
> > store file descriptors for the same file.  This allows the
> > client to open a file with different access modes (O_RDWR,
> > O_WRONLY, O_RDONLY) and add/remove the passed fds to/from an fd
> > set as needed.  This will allow QEMU to (in a later patch in this
> > series) "open" and "reopen" the same file by dup()ing the fd in
> > the fd set that corresponds to the file, where the fd has the
> > matching access mode flag that QEMU requests.
> >
> > The new QMP commands are:
> >    add-fd: Add a file descriptor to an fd set
> >    remove-fd: Remove a file descriptor from an fd set
> >    query-fdsets: Return information describing all fd sets
>
> > +
> > +# @AddfdInfo:
> > +#
> > +# Information about a file descriptor that was added to an fd set.
> > +#
> > +# @fdset_id: The ID of the fd set that @fd was added to.
> > +#
> > +# @fd: The file descriptor that was received via SCM rights and
> > +#      added to the fd set.
> > +#
> > +# Since: 1.2.0
>
> We're not very consistent on '1.2' vs. '1.2.0' in since listings, but
> that's probably worth a global cleanup closer to hard freeze.

I'll make a note of it.  Or does Luiz usually do a cleanup?

> > +##
> > +{ 'type': 'AddfdInfo', 'data': {'fdset_id': 'int', 'fd': 'int'} }
>
> This is a new command, so s/fdset_id/fdset-id/

Ok

> > +
> > +##
> > +# @add-fd:
> > +#
> > +# Add a file descriptor, that was passed via SCM rights, to an fd set.
> > +#
> > +# @fdset_id: #optional The ID of the fd set to add the file descriptor to.
> > +#
> > +# Returns: @AddfdInfo on success
> > +#          If file descriptor was not received, FdNotSupplied
> > +#          If @fdset_id does not exist, FdSetNotFound
> > +#
> > +# Notes: The list of fd sets is shared by all monitor connections.
> > +#
> > +#        If @fdset_id is not specified, a new fd set will be created.
> > +#
> > +# Since: 1.2.0
> > +##
> > +{ 'command': 'add-fd', 'data': {'*fdset_id': 'int'},
>
> Again, s/fdset_id/fdset-id/

Ok

> > +  'returns': 'AddfdInfo' }
> > +
> > +##
> > +# @remove-fd:
> > +#
> > +# Remove a file descriptor from an fd set.
> > +#
> > +# @fdset_id: The ID of the fd set that the file descriptor belongs to.
> > +#
> > +# @fd: #optional The file descriptor that is to be removed.
> > +#
> > +# Returns: Nothing on success
> > +#          If @fdset_id or @fd is not found, FdNotFound
> > +#
> > +# Since: 1.2.0
> > +#
> > +# Notes: The list of fd sets is shared by all monitor connections.
> > +#
> > +#        File descriptors that are removed:
> > +#        o will not be closed until the reference count corresponding
> > +#          to @fdset_id reaches zero.
> > +#        o will not be available for use after successful completion
> > +#          of the remove-fd command.
> > +#
> > +#        If @fd is not specified, all file descriptors in @fdset_id
> > +#        will be removed.
> > +##
> > +{ 'command': 'remove-fd', 'data': {'fdset_id': 'int', '*fd': 'int'} }
>
> And again.

Ok

> > +
> > +##
> > +# @FdsetFdInfo:
> > +#
> > +# Information about a file descriptor that belongs to an fd set.
> > +#
> > +# @fd: The file descriptor value.
> > +#
> > +# @removed: If true, the remove-fd command has been issued for this fd.
> > +#
> > +# Since: 1.2.0
> > +##
> > +{ 'type': 'FdsetFdInfo', 'data': {'fd': 'int', 'removed': 'bool'} }
>
> Is it worth providing any additional information?  For example, knowing
> whether the fd is O_RDONLY, O_WRONLY, or O_RDWR might be beneficial to
> management apps trying to discover what fds are already present after a
> reconnection, in order to decide whether to close them without having to
> resort to /proc/$qemupid/fdinfo/nnn lookups.  It might even be worth
> marking such information optional, present only when 'removed':false.

It makes sense but I'd like to limit the new functionality at this point 
so that I can get this support into QEMU 1.2.  Can this be added as a 
follow up patch?

> > +
> > +##
> > +# @FdsetInfo:
> > +#
> > +# Information about an fd set.
> > +#
> > +# @fdset_id: The ID of the fd set.
> > +#
> > +# @refcount: A count of the number of outstanding dup() references to
> > +#            this fd set.
> > +#
> > +# @in_use: If true, a monitor is connected and has access to this fd set.
> > +#
> > +# @fds: A list of file descriptors that belong to this fd set.
> > +#
> > +# Since: 1.2.0
> > +##
> > +{ 'type': 'FdsetInfo',
> > +  'data': {'fdset_id': 'int', 'refcount': 'int', 'in_use': 'bool',
> > +           'fds': ['FdsetFdInfo']} }
>
> s/fdset_id/fdset-id/; s/in_use/in-use/

Ok

> > +
> > +##
> > +# @query-fdsets:
> > +#
> > +# Return information describing all fd sets.
> > +#
> > +# Returns: A list of @FdsetInfo
> > +#
> > +# Since: 1.2.0
> > +#
> > +# Notes: The list of fd sets is shared by all monitor connections.
> > +#
> > +#        File descriptors are not closed until @refcount is zero,
> > +#        and either @in_use is false or @removed is true.
> > +#
> > +##
> > +{ 'command': 'query-fdsets', 'returns': ['FdsetInfo'] }
> > diff --git a/qerror.c b/qerror.c
> > index 92c4eff..63a0aa1 100644
> > --- a/qerror.c
> > +++ b/qerror.c
> > @@ -148,6 +148,10 @@ static const QErrorStringTable qerror_table[] = {
> >           .desc      = "No file descriptor supplied via SCM_RIGHTS",
> >       },
> >       {
> > +        .error_fmt = QERR_FDSET_NOT_FOUND,
> > +        .desc      = "File descriptor set with ID '%(id)' not found",
> > +    },
>
> Conflicts with Luiz's error cleanups.  I'm not sure whether libvirt
> needs to know this specific error, so I'd rather leave it generic for now.

Ok.  I'll try to use something more generic.

> > +++ b/qmp-commands.hx
> > @@ -926,6 +926,137 @@ Example:
> >
> >   EQMP
> >
> > +     {
> > +        .name       = "add-fd",
> > +        .args_type  = "fdset_id:i?",
> > +        .params     = "add-fd fdset_id",
>
> More fallout from s/_/-/ in the QMP naming, throughout this file.

Ok

--
Regards,
Corey