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



On Wed, Jun 27, 2012 at 11:16:32AM +0200, Kevin Wolf wrote:
> Am 27.06.2012 00:54, schrieb Eric Blake:
> >> It seems like libvirt would be in a better position to understand when a
> >> file is no longer in use, and then it can call close_fd.  No?  Of course
> >> the the only fd that needs to be closed is the originally passed fd. The
> >> dup'd fd's are closed by QEMU.
> > 
> > The more we discuss this, the more I'm convinced that commands like
> > 'block-commit' that will reopen a file to change from O_RDONLY to O_RDWR
> > will need to have an optional argument that says the name of the file to
> > reopen.  That is, I've seen three proposals:
> 
> Thanks for the summary. In fact, after having read this, I start
> thinking that we're oversimplifying things because we're only thinking
> about one use case, block-commit.
> 
> There are more: Live snapshot doesn't only put a new image on top, it
> must also make the old top-level image read-only. This isn't bad per se,
> but it shows that QMP commands can easily become bloated if you decide
> to change every command.
> 
> The really bad case that nobody thought of is that, when block-commit
> has finished, we need to switch back to read-only. This is an event that
> is not triggered by a certain monitor command, but that comes from
> inside qemu. I'm almost sure that we'll see more of this as we add more
> asynchronous commands.
> 
> This only works if we can pass the new file descriptor in advance. It
> would work nicely if you go with pass-fd and actually maintain a list of
> file descriptors for each /dev/fd/N, along with the different flags the
> file descriptors are meant for. I can't see how it would work with the
> temporary /dev/fdlist/N or the fd:name approach because they both imply
> that the original file descriptors are closed by the time that the QMP
> command returns.

What I don't understand though is that when you're "reopening" FDs, with
the pass-fd command approach, you're merely dup'ing the original FDs that
was passed in from the client.  Why can't you alternatively just dup the
FD you already have. I don't see why we need to keep the original FD
around forever.  If the QMP command handler nees the temporary /dev/fdlist/N
file to exist for longer than the duration of the command, it can simply
dup() it to get a permanent copy of its own.

Daniel
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