Re: [Xen-devel] [PATCH RFC] xen-block: correctly define structures in public headers

[Ian Campbell <Ian.Campbell@citrix.com>]

On Tue, 2013-12-03 at 15:11 +0000, David Vrabel wrote:
> On 03/12/13 13:41, Ian Campbell wrote:
> > On Tue, 2013-12-03 at 11:59 +0000, David Vrabel wrote:
> >> On 03/12/13 11:08, Ian Campbell wrote:
> >>> On Tue, 2013-12-03 at 11:01 +0000, David Vrabel wrote:
> >>>> On 03/12/13 10:57, Roger Pau Monne wrote:
> >>>>> Using __packed__ on the public interface is not correct, this
> >>>>> structures should be compiled using the native ABI, and __packed__
> >>>>> should only be used in the backend counterpart of those structures
> >>>>> (which needs to handle different ABIs).
> >>>>>
> >>>>> This was even worse in the ARM case, where the Linux kernel was
> >>>>> incorrectly using the X86_32 protocol ABI. This patch fixes it, but
> >>>>> also breaks compatibility, so an ARM DomU kernel compiled with
> >>>>> this patch will fail to communicate with PV disk devices unless the
> >>>>> Dom0 also has this patch.
> >>>>
> >>>> This ABI change needs to be justified.  Why do you think it is
> >>>> acceptable to break existing Linux guests?  Because I don't think it is.
> >>>
> >>> As I explained in my reply those guests are buggy.
> >>
> >> The kernel has a strong policy on not changing ABIs, even to fix bugs.
> >> I don't think a bug fix alone is sufficient justification for ABI breakage.
> > 
> > This is not the kernels ABI to fix in stone. The ABI is defined by the
> > upstream Xen project and Linux has failed to follow the specified ABI
> > correctly.
> 
> I disagree. The working back and front end implementations have created
> a new, different ABI.

Which is completely incompatible with every other OS. This issue was
discovered trying to run a NetBSD guest on a Linux dom0.

I will not legitimize this broken ABI because then all these other OSes
would need to implement all of this compatibility cruft in order to
interoperate with Linux.

> > There is no deployed legacy of guests to support here.
> 
> I not sure I agree. It does seem to be widely used and I'm not sure we
> have sufficient visibility on how Xen on Arm is being used to know if
> this change will cause other people problems.

It's being widely developed on and used for PoC etc. It is not widely
deployed.

> If Konrad and Boris agree that breaking the kernel's ABI in this way is
> acceptable in this specific case, I'll defer to them.

My opinion as Xen on ARM hypervisor maintainer is that this is the right
thing to do in this case.

Ian.