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