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

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.