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Re: [libvirt] [PATCH v3 0/4] command line fd passing using fd sets

On 10/16/2012 11:51 AM, Corey Bryant wrote:
> This series adds command line file descriptor passing support
> via a new -add-fd option.  This is a follow-on to the existing
> QMP fd passing support provided in the following patch series:
> comments.gmane.org/gmane.comp.emulators.qemu/165463
> The new -add-fd option is designed to mirror the add-fd QMP
> option as much as possible.

Before this can still be useful in libvirt, we need a way to pass in fds
for all of a backing chain, and not just the top-level.  And you can't
argue that libvirt should be temporarily rewriting qcow2 metadata to
stick in names such as /dev/fdset/1 into the metadata.  Consider the
case of:

                        /- guest1.img
base.img <- snap1.img <
                        \- guest2.img

snap1.img is supposed to be read-only, and there's no guarantee that
/dev/fdset/1 will be available for both guest1 and guest2 to rewrite the
header.  Thus, I argue that snap1.img must encode the name base.img in
its metadata, but that we must also have a way to tell qemu to open
/dev/fdset/1 instead of trying open(base.img) in order to get the open
fd for base.img.

But even though we are still not at a point where the overall feature is
useful, I can at least review these patches for getting us incrementally
closer to our goal.

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

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