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

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Mon, 29 Sep 2014, Wei Liu wrote:
> 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.

No, the flag would only make kernels >= 3.17 work if they are built with
CONFIG_ARM_LPAE.



> > 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.

We are trying to find alternative solutions to the problem. I can't say
it is easy but I am working on an alternative approach.


> 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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