Re: [Qemu-devel] [libvirt] [PATCH v2] Add support for fd: protocol

[Corey Bryant <bryntcor@us.ibm.com>]

On 06/14/2011 04:18 PM, Eric Blake wrote:
> On 06/14/2011 01:55 PM, Corey Bryant wrote:
>>> >>  So we would need something like -drive
>>> >>  file=fd:4,format=qcow2,backing=fd:5
>>> >>
>>> >>  and since backing files can be nested, we'd need some way of specifying
>>> >>  more than one level of backing file.  Libvirt already knows how to walk
>>> >>  a chain of backing images in qcow2 files (it has to, in order to set
>>> >>  sVirt SELinux permissions on all of those files so that qemu can open()
>>> >>  each file), so it wouldn't be much harder for libvirt to instead do the
>>> >>  open() and pass each fd.
>>> >>
>> >  Right.  So what I was talking about here (and poorly stated) is that
>> >  starting Qemu with a copy-on-write image file causes a reopen of the
>> >  backing file. In this case you would only be passing the fd of the
>> >  copy-on-write file to Qemu. I've fenced that off as unsupported for now.
>> >
>> >  I like your approach for passing multiple fds. Do you think backing file
>> >  support is needed immediately with this patch?
> Incremental support is fine by me, but we'll need it sooner or later.
> And for the nested backing file case, does:
>
> -drive file=fd:4,format=qcow2,backing=fd:5,backing=fd:6
>
> work, or would we need:
>
> -drive file=fd:4,format=qcow2,backing1=fd:5,backing2=fd:6
>
> That is, can you have more than one backing=, or do you need distinct
> backingN=, and if you have to support multiple N, we must make sure the
> solution allows arbitrarily deep nesting (or at least as deep as any
> current limits qemu places on qcow2 backing file nesting).

Qemu currently opens the specified copy-on-write file, finds a backing 
file name in the opened file's header, opens that backing file, and repeats.

So it seems that either a backingN= would be needed or a way to map a 
file descriptor to the file name in the header.

I think the latter is along the lines of what was discussed here with 
-blockdev:
http://lists.gnu.org/archive/html/qemu-devel/2011-05/msg02417.html

Repeating the syntax specified in that thread, you'd have:

-blockdev id=foo-base,path=fd:4,format=raw
-blockdev id=foo,path=fd:3,format=qcow2,backing_file=foo-base

where id is the file name.  This chains the backing files on the command 
line.

Corey