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

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