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

[Juergen Gross <jgross@xxxxxxxx>]

On 10/04/17 17:32, Raslan, KarimAllah wrote:
> 
> Ahmed, Karim Allah
> karahmed@xxxxxxxxx
> 
> 
> 
>> On Apr 10, 2017, at 3:57 PM, Juergen Gross <jgross@xxxxxxxx> wrote:
>>
>> On 10/04/17 15:47, Boris Ostrovsky wrote:
>>> On 04/07/2017 06:11 PM, Stefano Stabellini wrote:
>>>> On Fri, 7 Apr 2017, Boris Ostrovsky wrote:
>>>>> On 04/07/2017 01:36 PM, Stefano Stabellini wrote:
>>>>>> On Fri, 7 Apr 2017, Boris Ostrovsky wrote:
>>>>>>> On 04/07/2017 07:58 AM, Ian Jackson wrote:
>>>>>>>> tl;dr:
>>>>>>>> Please apply
>>>>>>>>
>>>>>>>>    da72ff5bfcb02c6ac8b169a7cf597a3c8e6c4de1
>>>>>>>>    partially revert "xen: Remove event channel notification through
>>>>>>>>      Xen PCI platform device"
>>>>>>>>
>>>>>>>> to all stable branches which have a version of the original broken
>>>>>>>> commit.  This includes at least 4.9.y.
>>>>>>>>
>>>>>>>> Background:
>>>>>>>>
>>>>>>>> osstest service owner writes ("[linux-4.9 baseline test] 107238: 
>>>>>>>> tolerable FAIL"):
>>>>>>>> ...
>>>>>>>>> test-amd64-amd64-qemuu-nested-intel 13 xen-boot/l1  fail never pass
>>>>>>>> osstest doesn't consider this a regresion because it looks for
>>>>>>>> regressions within a branch, and this is the first test of Linux 4.9.
>>>>>>>> However, this is a regression from the kernel we are currently using.
>>>>>>>>
>>>>>>>> L1 dom0 console log:
>>>>>>>>  
>>>>>>>> http://logs.test-lab.xenproject.org/osstest/logs/107238/test-amd64-amd64-qemuu-nested-intel/huxelrebe0---var-log-xen-osstest-serial-l1.guest.osstest.log
>>>>>>>>
>>>>>>>> It seems to have got stuck halfway through booting.
>>>>>>>>
>>>>>>>> The message
>>>>>>>>  (XEN) *** Serial input -> Xen (type 'CTRL-x' three times to switch 
>>>>>>>> input to DOM0)
>>>>>>>> shows where osstest timed out on this test, and started its log
>>>>>>>> capture process (including collecting debug key output).
>>>>>>>>
>>>>>>>> Complete logs for this job here:
>>>>>>>>  
>>>>>>>> http://logs.test-lab.xenproject.org/osstest/logs/107238/test-amd64-amd64-qemuu-nested-intel/info.html
>>>>>>>>
>>>>>>>> Juergen Gross tells me that this is due to the lack of
>>>>>>>> da72ff5bfcb02c6ac8b169a7cf597a3c8e6c4de1.
>>>>>>>>
>>>>>>>> Thanks,
>>>>>>>> Ian.
>>>>>>>>
>>>>>>>> PS: Stefano, Boris: did you already request a backport of this commit?
>>>>>>>> If not, why not ?
>>>>>>> No, but this should indeed be backported to 4.9+
>>>>>> Boris, are you going to do that?
>>>>> Is there anything that needs to be done beyond just applying it to 4.9
>>>>> (4.10 apparently already has it).
>>>> No, I don't think so. 4.9 already has the offending commit.
>>>
>>>
>>> Looks like there will be a new version of the original patch
>>> (72a9b186292) so we should hold off with backport request to 4.9:
>>>
>>> https://lists.xenproject.org/archives/html/xen-devel/2017-04/msg01468.html
>>
>> TBH: I'm not convinced by the reasoning why 72a9b186292 has to be
>> reworked: Do we really care for Xen versions < 4.0 and a theoretical
>> problem (after all the author admitted the bug isn't being hit in
>> reality due to a short-circuit in the code)?
> 
> IMHO, even if 72a9b186292 has not been reworked we should completely revert it
> not only partially revert it. Before this commit at least kernel 4.9+ would
> work on older Xen versions (< 4.0) while now, it will not even boot.

The following is meant as a real question without any prejudice:

How old a Xen version do we want to support in the Linux kernel?

- Only those being still maintained (meaning getting security fixes)
- Versions max. X years after getting last security fixes (what value
  of X?)
- From version Y on (what value of Y?)
- All versions which we can think of

I think we should answer this question in order to know what we can
remove in the Linux kernel without breaking something.

> I do agree however that fixing INTx sounds completely useless since there is
> no combination of Xen+Linux that would lead to the bug by default (unless you
> forced the use of INTx even when vector injection is supported which is what I
> did during testing the original patch).

Right.


Juergen

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