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.