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

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

I think I might have confused remove-fd and close-fdset in earlier emails in this thread, so I hope this isn't inconsistent with what I said before.

> 6. qemu_close (need to replace all close calls in block layer with > qemu_close) > -> qemu_close decrements refcount for fdset > -> qemu_close closes all fds that have (refcount == 0 && (!inuse || remove)) > -> qemu_close frees the fdset if no fds remain in it > > 7. disconnecting the QMP monitor > -> monitor disconnect visits all fdsets on monitor and turns off monitor > in-use flag for fdset And close all fds with refcount == 0.

> 8. connecting the QMP monitor > -> monitor connect visits all fdsets on monitor and turns on monitor > in-use flag for fdset > > 9. query-fdsets > -> returns all fdsets and fds that don't have remove flag on