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Re: [Qemu-devel] [RFC PATCH 0/4] block: file descriptor passing using -f
From: |
Eric Blake |
Subject: |
Re: [Qemu-devel] [RFC PATCH 0/4] block: file descriptor passing using -filefd and getfd_file |
Date: |
Tue, 22 May 2012 09:01:45 -0600 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:12.0) Gecko/20120430 Thunderbird/12.0.1 |
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 address@hidden +1-919-301-3266
Libvirt virtualization library http://libvirt.org
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- [Qemu-devel] [RFC PATCH 2/4] qmp/hmp: Add getfd_file monitor command, (continued)
- [Qemu-devel] [RFC PATCH 2/4] qmp/hmp: Add getfd_file monitor command, Corey Bryant, 2012/05/21
- Re: [Qemu-devel] [RFC PATCH 0/4] block: file descriptor passing using -filefd and getfd_file, Kevin Wolf, 2012/05/22
- Re: [Qemu-devel] [RFC PATCH 0/4] block: file descriptor passing using -filefd and getfd_file, Eric Blake, 2012/05/22
- Re: [Qemu-devel] [RFC PATCH 0/4] block: file descriptor passing using -filefd and getfd_file, Corey Bryant, 2012/05/22
- Re: [Qemu-devel] [RFC PATCH 0/4] block: file descriptor passing using -filefd and getfd_file, Corey Bryant, 2012/05/22
- Re: [Qemu-devel] [RFC PATCH 0/4] block: file descriptor passing using -filefd and getfd_file, Kevin Wolf, 2012/05/22
- Re: [Qemu-devel] [RFC PATCH 0/4] block: file descriptor passing using -filefd and getfd_file, Corey Bryant, 2012/05/22
- Re: [Qemu-devel] [RFC PATCH 0/4] block: file descriptor passing using -filefd and getfd_file, Eric Blake, 2012/05/22
- Re: [Qemu-devel] [RFC PATCH 0/4] block: file descriptor passing using -filefd and getfd_file, Corey Bryant, 2012/05/22