Re: [Qemu-devel] [PATCH v4 0/7] file descriptor passing using pass-fd

[Luiz Capitulino <lcapitulino@redhat.com>]

On Mon, 09 Jul 2012 17:46:00 +0200
Kevin Wolf <kwolf@redhat.com> wrote:

> Am 09.07.2012 17:05, schrieb Corey Bryant:
> > I'm not sure this is an issue with current design.  I know things have 
> > changed a bit as the email threads evolved, so I'll paste the current 
> > design that I am working from.  Please let me know if you still see any 
> > issues.
> > 
> > FD passing:
> > -----------
> > New monitor commands enable adding/removing an fd to/from a set.  New 
> > monitor command query-fdsets enables querying of current monitor fdsets. 
> >   The set of fds should all refer to the same file, with each fd having 
> > different access flags (ie. O_RDWR, O_RDONLY).  qemu_open can then dup 
> > the fd that has the matching access mode flags.
> > 
> > Design points:
> > --------------
> > 1. add-fd
> > -> fd is passed via SCM rights and qemu adds fd to first unused fdset 
> > (e.g. /dev/fdset/1)

The fdset should be specified by the client, like:

 { "execute": "add-fd-set", "arguments": { "set-name": "/dev/fdset/1" } }

> > -> add-fd monitor function initializes the monitor inuse flag for the 
> > fdset to true

Why do we need the inuse flag?

> > -> add-fd monitor function initializes the remove flag for the fd to false
> > -> add-fd returns fdset number and received fd number (e.g fd=3) to caller
> > 
> > 2. drive_add file=/dev/fdset/1
> > -> qemu_open uses the first fd in fdset1 that has access flags matching 
> > the qemu_open action flags and has remove flag set to false
> > -> qemu_open increments refcount for the fdset
> > -> Need to make sure that if a command like 'device-add' fails that 
> > refcount is not incremented
> > 
> > 3. add-fd fdset=1
> > -> fd is passed via SCM rights
> > -> add-fd monitor function adds the received fd to the specified fdset 
> > (or fails if fdset doesn't exist)
> > -> add-fd monitor function initializes the remove flag for the fd to false
> > -> add-fd returns fdset number and received fd number (e.g fd=4) to caller
> > 
> > 4. block-commit
> > -> qemu_open performs "reopen" by using the first fd from the fdset that 
> > has access flags matching the qemu_open action flags and has remove flag 
> > set to false
> > -> qemu_open increments refcount for the fdset
> > -> Need to make sure that if a command like 'block-commit' fails that 
> > refcount is not incremented
> > 
> > 5. remove-fd fdset=1 fd=4
> > -> remove-fd monitor function fails if fdset doesn't exist
> > -> remove-fd monitor function turns on remove flag for fd=4
> 
> What was again the reason why we keep removed fds in the fdset at all?
> 
> The removed flag would make sense for a fdset after a hypothetical
> close-fdset call because the fdset needs to be kept around until the
> last user closes it, but I think removed fds can be deleted immediately.

Agreed.