On Mon, 09 Jul 2012 13:35:19 -0400 Corey Bryant <coreyb@xxxxxxxxxxxxxxxxxx> wrote: > > > On 07/09/2012 11:46 AM, Kevin Wolf 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) > >> -> 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? > > Because if refcount is > 0 for the fd set, then the fd could be in use > by a block device. So we keep it around until refcount is decremented > to zero, at which point it is safe to close. But then the refcount is associated with the set, not with any particular fd. -- libvir-list mailing list libvir-list@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/libvir-list