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Re: [Xen-devel] Xen Project policy on feature flags

On Mon, Sep 29, 2014 at 10:36:35AM +0100, George Dunlap wrote:
> On 09/29/2014 10:31 AM, Wei Liu wrote:
> >On Mon, Sep 29, 2014 at 10:00:13AM +0100, George Dunlap wrote:
> >>On 09/26/2014 03:49 PM, Stefano Stabellini wrote:
> >>>>Let me rephrase - will it boot in the same fashion (And with the same
> >>>>bugs) as it did prior to this functionality being introduced?
> >>>3.15 -> dom0 on ARM broken (if netback is used)
> >>>3.17 -> dom0 on ARM is fixed, only if the kernel is compiled with 
> >>>
> >>>Reverting the XENFEAT_grant_map_identity related changes would give you
> >>>a system broken even with CONFIG_ARM_LPAE.
> >>>Reverting Zoltan's changes to netback would give you a working system.
> >FWIW reverting isn't practical as many more fixes have gone in.
> >
> >I think a possible workaround is to copy directly xen-netback directory
> >from 3.14 and build it against new kernel. Netback itself is quite
> >self-contained.
> Could we provide a patch which would just disable the problematic behavior?

Unfortunately no. Zoltan removed the other data path (copying from guest
to Dom0, which works for ARM) completely. It's not something as simple
as flipping a switch in netback to bring it back.

> >
> >>So the *only* reason to include this flag at this point is to allow people
> >>to run an unmodified 3.17 kernel with netback, is that right?  It seems like
> >>a really jerk move to encourage people to put 3.17 on their systems, and
> >>then deliberately break it on a subsequent release.
> >>
> >I think this applies to all netback > 3.15. That means all unmodified
> >kernels released during our next release cycle.
> Well adding this flag won't make 3.15 and 3.16 work, will it?  Or do you

As I understand it, no, it won't make 3.15 and 3.16 work.

> expect the code to use this flag to trickle back to those kernels as well?

This one is for Stefano.  I can't say for sure but that looks unlikely
to me.


> In any case, what that means all those kernels would be broken should we
> ever decide to remove the flag.  I think it's much better to ask someone to
> use an older kernel or apply a patch.
>  -George

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