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

On Mon, 09 Jul 2012 13:35:19 -0400 Corey Bryant <coreyb(a)linux.vnet.ibm.com&gt; 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.