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

[Kevin Wolf <kwolf@redhat.com>]

Am 07.08.2012 18:49, schrieb Eric Blake:
> 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.
> 
>> +##
>> +{ 'type': 'AddfdInfo', 'data': {'fdset_id': 'int', 'fd': 'int'} }
> 
> This is a new command, so s/fdset_id/fdset-id/
> 
>> +
>> +##
>> +# @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/
> 
>> +  '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.
> 
>> +
>> +##
>> +# @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.

Why do we even include removed=true file descriptors in query-fdsets?
Shouldn't they appear to be, well, removed from a clients POV?

The problem with adding flags is the same as with errno numbers: How to
do it in a platform independent way? The management application might
run on a different OS than qemu, so a numeric 'flags' field could have
an entirely different meaning there.

We could add bools for some flags and an enum for
O_RDONLY/O_WRONLY/O_RDWR, but it's probably better to wait until we know
which of them we really need.

Kevin