Re: [libvirt] [Qemu-devel] [RFC PATCH 0/4] block: file descriptor passing using -filefd and getfd_file
On 05/22/2012 08:45 AM, Kevin Wolf wrote:
> I understand that open("/dev/fd/42") would be the same
as dup(42), but
> I'm not sure that I'm entirely clear on how this would work. Could you
> give an example?
With your approach you open the file outside qemu, pass the fd to qemu
along with a file name that it's supposed to replace and then you use
that fake file name:
(qemu) getfd_file abc
(qemu) drive_add 0 file=abc,...
Instead you could use the existing getfd command and avoid the translation:
(qemu) getfd
42
(qemu) drive_add 0 file=/dev/fd/42,...
Er, well. Just that getfd doesn't return the assigned fd today, so the
management tool doesn't know it. We would have to add that.
That actually sounds workable. As long as management knows _what_ fd
qemu recieved (that is, 'getfd' is enhanced to tell libvirt the
associated fd number), and as long as qemu makes the magic naming of
/dev/fd/ work everywhere (even if it isn't normally part of the host
OS), then libvirt could indeed reuse existing file mechanisms to open a
file using an fd that it knows qemu should already own, without needing
to invent a new 'getfd_file' monitor command. I guess in this instance,
libvirt would have to unconditionally use 'closefd' after the command
that reused the fd, since using file=/dev/fd/42 dups the fd rather than
consuming it (this is different from commands that use fd:nnn to consume
an fd).
--
Eric Blake eblake(a)redhat.com +1-919-301-3266
Libvirt virtualization library http://libvirt.org
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