Re: [libvirt] [PATCH v3 0/5] file descriptor passing using pass-fd

On 06/20/2012 02:31 AM, Daniel P. Berrange wrote: >> This might mean that libvirt should only closefd the file when it >> becomes unused (like after hot unplug); or that qemu must keep it open >> internally even after closefd as long as the block device is still in use. > > As it works today, the only time libvirt would call "closefd", is if > the monitor command it was trying to use the FD with (eg drive_add) > failed. If drive_add was successfully run, then libvirt would not be > invoking closefd. But right now, the only time libvirt uses 'getfd' is with commands like 'migrate' that implicitly close the fd after it is used by name; we don't have any experience in using fds by '/dev/fd/nn' notation instead of name. It is the fact that /dev/fd/nn will allow us to use 'pass-fd' in more situations in before, by dup()ing the fd, that libvirt all the sudden becomes responsible for using 'closefd' at the appropriate moments. I guess I can live with a rule that libvirt must not call 'closefd' on any fd that it might later want to reassign to a new copy of the fd; that is, with backing chains, if libvirt originally calls 'pass-fd drive-virtio1' with an O_RDONLY fd and gets back 21, then libvirt must not call 'closefd drive-virtio1' until it knows drive-virtio1 is no longer needed. Converting the 'drive-virtio1' fd to O_RDWR while still keeping it at /dev/fd/21 could be done via 'pass-fd -f drive-virtio1', where -f is an optional bool parameter to force a reassociation of a given name back to the previously assigned value instead of the normal error path for accidentally passing an fd to an already in-use name.