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

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

On 06/14/2011 12:13 PM, Eric Blake wrote:
> On 06/14/2011 07:31 AM, Corey Bryant wrote:
>> >     - Starting Qemu with a backing file
> What do you mean by this?  Taking a guess:
>
> In the case of a qcow2 image with a backing file, does that mean that
> both the qcow2 image and it's backing file can both be passed to qemu
> via fd: notations?  That is, if the -drive file=fd:4,format=qcow2 option
> is passed, and fd4 is a qcow2 image that also has a backing file, it
> seems like libvirt should also be able to pass that backing file via
> another fd, so that qemu doesn't have to open() the backing file
> directly.  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.
>
> -- Eric Blake eblake@redhat.com +1-801-349-2682 Libvirt virtualization
> library http://libvirt.org

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?