Re: Fixing libvirt's libxl driver breakage -- where to define LIBXL_API_VERSION?

From: Jim Fehlig <jfehlig@suse.com>
To: George Dunlap <george.dunlap@citrix.com>
Cc: LibVirt Development List <libvir-list@redhat.com>,
	Xen-devel <xen-devel@lists.xenproject.org>,
	Wei Liu <wei.liu2@citrix.com>,
	Ian Jackson <Ian.Jackson@eu.citrix.com>
Subject: Re: Fixing libvirt's libxl driver breakage -- where to define LIBXL_API_VERSION?
Date: Wed, 13 Apr 2016 17:31:00 -0600	[thread overview]
Message-ID: <570ED6B4.3060102__18350.0236053534$1460590360$gmane$org@suse.com> (raw)
In-Reply-To: <CAFLBxZbXDwPMXJT7q4CqEdfQS=nHgphzaysTmUQ-2bqXnGpfTg@mail.gmail.com>

On 04/13/2016 03:09 AM, George Dunlap wrote:
> On Tue, Apr 12, 2016 at 10:31 PM, Jim Fehlig <jfehlig@suse.com> wrote:
>> Wei Liu wrote:
>>> Hi libvirt maintainers,
>> Sorry for the delay. Slowly catching up on mail after vacation...
>>
>>> Xen's control library libxenlight (libxl) requires application
>>> (libvirt in this case) to explictily define LIBXL_API_VERSION.
>> Where is this requirement written? :-)
>>
>>> This is
>>> lacking at the moment so libvirt's libxl driver always gets the latest
>>> APIs.
>> IMO, that is what we want for upstream libvirt. Downstreams can choose a
>> specific version if they want.
> I think one of us isn't understanding the situation properly. Is it
> not the case that currently, all releases of libvirt *will not
> compile* against Xen 4.7 once it's released?  So people downloading
> and building libvirt will have to either 1) root around and try to
> figure out what version of Xen it will build against, 2) manually add
> in a #define *with the correct API version* to a header somewhere to
> make it build properly, or 3) update to a version of libvirt that
> supports the new api (which at the moment hasn't even been released
> yet)?
>
> All of those options are completely unacceptable.  Older versions of
> libvirt should Just Work when compiled against newer versions of Xen.

Yes, agreed. In practice though folks want a new libvirt with the new Xen, which
is probably why no one has complained thus far.

I'll knock up a patch to set the LIBXL_API_VERSION to 0x040200. The only APIs
that have changed since 0x040200 are libxl_set_vcpuaffinity and
libxl_domain_create_restore, but libvirt does not use the changes (added
params). libvirt does use new libxl APIs added since Xen 4.2, but those aren't
tied to a specific LIBXL_API_VERSION.

Regards,
Jim

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