On Thu, May 17, 2012 at 08:14:15AM -0600, Eric Blake wrote:
On 05/17/2012 07:42 AM, Stefan Hajnoczi wrote:
The -open-hook-fd approach allows QEMU to support file descriptor passing without changing -drive. It also supports snapshot_blkdev and other commands
By the way, How will it support them?
The problem with snapshot_blkdev is that closing a file and opening a new file cannot be done by the QEMU process when an SELinux policy is in place to prevent opening files.
snapshot_blkdev can take an fd:name instead of a /path/to/file for the file to open, in which case libvirt can pass in the named fd _prior_ to the snapshot_blkdev using the 'getfd' monitor command.
The -open-hook-fd approach works even when the QEMU process is not allowed to open files since file descriptor passing over a UNIX domain socket is used to open files on behalf of QEMU.
The -open-hook-fd approach would indeed allow snapshot_blokdev to ask for the fd after the fact, but it's much more painful. Consider a case with a two-disk snapshot:
with the fd:name approach, the sequence is:
libvirt calls getfd:name1 over normal monitor qemu responds libvirt calls getfd:name2 over normal monitor qemu responds libvirt calls transaction around blockdev-snapshot-sync over normal monitor, using fd:name1 and fd:name2 qemu responds
but with -open-hook-fd, the approach would be:
libvirt calls transaction qemu calls open(file1) over hook libvirt responds qemu calls open(file2) over hook libvirt responds qemu responds to the original transaction
The 'transaction' operation is thus blocked by the time it takes to do two intermediate opens over a second channel, which kind of defeats the purpose of making the transaction take effect with minimal guest downtime. And libvirt code becomes a lot trickier to deal with the fact that two channels are in use, and that the channel that issued the 'transaction' command must block while the other channel for handling hooks must be responsive.
I'm really disliking the hook-fd approach, when a better solution is to make use of 'getfd' in advance of any operation that will need to open new fds.
This is a good technical argument for using getfd. I agree with you. Stefan