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

[Wei Liu <wei.liu2@xxxxxxxxxx>]

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 
> >>>CONFIG_ARM_LPAE
> >>>
> >>>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.

Wei.

> 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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