Re: [Qemu-devel] [libvirt] [RFC 0/5] block: File descriptor passing using -open-hook-fd

[Eric Blake <eblake@redhat.com>]

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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.

-- 
Eric Blake   eblake@redhat.com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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