Re: Xen Project policy on feature flags

[David Vrabel <david.vrabel@citrix.com>]

On 29/09/14 10:36, 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?

No.  This would require re-introducing the grant copy from-guest path to
netback.  This would be expensive since netback has seen significant
changes since (multi-queue support in particular).

It would also not fix the underlying ARM-specific bug and other users of
grant mapping would be similarly broken.

I think we should:

1. Revert XENFEAT_grant_map_identity.
2. Add the flush-cache-by-bus-address hypercall.
3. Add the Linux support this this cache operation and tag this for stable.
4. Backport the hypercall to 4.4.x.

I think this is critical to fix in 4.5 and should have a freeze
exception.  I would even consider slipping the 4.5 release to get this
fixed.

David