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

[Juergen Gross <jgross@xxxxxxxx>]

On 11/04/17 16:42, Raslan, KarimAllah wrote:
> 
>> 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.

Thanks for this summary. With this information I agree reverting is the
best option.

>>> 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.

Yes, please go ahead.


Juergen

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