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Re: [libvirt] [Qemu-devel] live snapshot wiki updated



"Daniel P. Berrange" <berrange redhat com> writes:

> On Tue, Jul 19, 2011 at 04:14:27PM +0100, Stefan Hajnoczi wrote:
>> On Tue, Jul 19, 2011 at 3:30 PM, Jes Sorensen <Jes Sorensen redhat com> wrote:
>> > On 07/19/11 16:24, Eric Blake wrote:
>> >> [adding the libvir-list]
>> >> On 07/19/2011 08:09 AM, Jes Sorensen wrote:
>> >>> Urgh, libvirt parsing image files is really unfortunate, it really
>> >>> doesn't give me warm fuzzy feelings :( libvirt really should not know
>> >>> about internals of image formats.
>> >>
>> >> But even if you add new features to qemu to avoid needing this in the
>> >> future, it doesn't change the past - libvirt will always have to know
>> >> how to parse image files understood by older qemu, and so as long as
>> >> libvirt already knows how to do that parsing, we might as well take
>> >> advantage of it.
>> >
>> > What has been done here in the past is plain wrong. Continuing to do it
>> > isn't the right thing to do here.
>> >
>> >> Besides, I feel that having a well-documented file format, so that
>> >> independent applications can both parse the same file with the same
>> >> semantics by obeying the file format specification, is a good design goal.
>> >
>> > We all know that documentation is rarely uptodate, new features may not
>> > get added and libvirt will never be able to keep up. The driver for a
>> > file format belongs in QEMU and nowhere else.
>> 
>> It should be a goal to avoid dependencies in multiple layers of the
>> stack because it becomes are to add new features - they require
>> coordinated changes in multiple layers.  Having both QEMU and libvirt
>> know the internals of image files is such a multi-dependency.  If I
>> want to add a new format or change an existing format I have to touch
>> both layers.
>> 
>> For fd-passing perhaps we have an opportunity to use a callback
>> mechanism (QEMU request: filename -> libvirt response: fd) and do all
>> the image format parsing in QEMU.
>
> The reason why libvirt does the parsing of file headers to determine
> backing files is to maintain the trust boundary. Everything run from
> the exec() of QEMU onwards is considered untrusted code. So having
> QEMU parsing the file headers & passing back open() requests to libvirt
> is breaking the trust boundary.

Exactly.

The block drivers form a tree.  Each driver opens its children.  For
convenience, some of them have information on how to open their children
encoded in their image (e.g. COW backing images).  Others receive it as
configuration, encoded in their "filename" string (e.g. blkdebug).

Thus, we have direct control only over the root block driver.  We can
pass it fds easily.

We control the non-root block drivers only indirectly, through the block
drivers along the path from the root.  We don't have a way to pass them
fds.

If we had a way to construct the tree bottom-up, we could directly
control all the block drivers.

Requires knowing the number of children, and extracting child
configuration from images where applicable.

[...]


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