[libvirt] [Qemu-devel] [RFC PATCH 1/4] qemu-options: Add -filefd command line option

[Stefan Hajnoczi]

On Tue, May 22, 2012 at 2:38 PM, Kevin Wolf <kwolf at redhat.com> wrote:
> Am 22.05.2012 15:25, schrieb Corey Bryant:
>>
>>
>> On 05/21/2012 05:40 PM, Eric Blake wrote:
>>> On 05/21/2012 02:19 PM, Corey Bryant wrote:
>>>> This patch provides support for the -filefd command line option.
>>>> This option will allow passing of a filename and its corresponding
>>>> file descriptor to QEMU at exec time.
>>>>
>>>> Signed-off-by: Corey Bryant<coreyb at linux.vnet.ibm.com>
>>>
>>>> +DEF("filefd", HAS_ARG, QEMU_OPTION_filefd,
>>>> +    "-filefd file=<filename>,fd=<fd>\n"
>>>
>>> I take it that if filename contains ',', then we have to escape it on
>>> the command line?  Is it worth passing fd first and file second by
>>> default, as a possible way to avoid the need for escaping, or does the
>>> option parser not care about ordering?
>>>
>>
>> That's a good question.  The options can be ordered either way so I
>> don't think we'll force fd to be specified first.  I imagine this should
>> behave no differently than "-drive file=xyz,if=none,...".  I ran a quick
>> test using -drive with a filename that had a comma, and (escaped or not)
>> it failed on the option parsing.  So it looks like if you have a path
>> with a comma you're not going to have any luck.
>
> I think you can escape it, you'd have to use a double comma. But I'd
> rather not introduce more of this. It's another good reason for using
> /dev/fd/... instead of a translation table.

I'm not sure how this solves backing file descriptor passing?  How
will QEMU know to use /dev/fd/10 for a backing file that is referenced
from /dev/fd/9 as "backing.img"?  (There was discussion about
rewriting backing filenames but I think that solution is risky and we
should avoid it.)

Stefan