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

[Corey Bryant <coreyb@linux.vnet.ibm.com>]

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