Re: [Xen-devel] Please apply "partially revert "xen: Remove event channel..."

["Raslan, KarimAllah" <karahmed@xxxxxxxxx>]

> On Apr 11, 2017, at 4:10 PM, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx> 
> wrote:
> 
> 
> 
>>> 
>>> I think the right thing is indeed to revert 72a9b186292 (and
>>> therefore da72ff5bfcb02).  Any objections?
>> 
>> For the end result: depends. Is there a real error or not?
>> KarimAllah wrote that his concerns are of a theoretical nature as
>> xen_strict_xenbus_quirk() would mask the problem. OTOH he tells us
>> a 4.9 kernel wouldn't even boot on Xen < 4.0. What is correct here?
> 
> 
> Judged by 'BUG_ON(!xen_feature(XENFEAT_hvm_callback_vector))' in 
> xen_hvm_guest_init() this can't boot on 3.4.

Correct.

Here is the brief summary of the current situation:

Before the offending commit (72a9b186292):

1) INTx does not work because of the reset_watches path.
2) The reset_watches path is only taken if you have Xen > 4.0
3) The Linux Kernel by default will use vector inject if the hypervisor
   support. So even INTx does not work no body running the kernel with Xen > 4.0
   would notice. Unless he explicitly disabled this feature either in the kernel
   or in Xen (and this can only be disabled by modifying the code, not
   user-supported way to do it).

After the offending commit (+ partial revert):

1) INTx is no longer support for HVM (only for PV guests).
2) Any HVM guest The kernel will not boot on Xen < 4.0 which does not have
   vector injection support. Since the only other mode supported is INTx which.

So based on this summary, I think before commit (72a9b186292) we were in much
better position from a user point of view.

> 
> 
>> 
>> For just reverting the two commits: yes, as there would be conflicts
>> with already applied patches, especially the pv isolation patches by
>> Vitaly and pvh v1 removal.
>> 
>> So in case we need a revert I'd ask KarimAllah to send a fixup patch
>> restoring the state before 72a9b186292 while respecting the new
>> structure to be found on the for-linus-4.12 branch of xen/tip.
> 
> Stable trees (4.9 and 4.10) need a pure revert. 4.11 indeed requires some 
> extra work (and 4.12 is even more involved).

If we agreed on going forward with the revert, I will take care of sending the
patches to revert for various trees.

> 
> -boris
> 

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