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

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

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.

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).

> 
> And even if we do: I'd rather add another patch to stable later than
> keeping a real bug in Linux 4.9 which has been hit at least 3 times
> up to now (by Stefano, George and Ian).
> 
> 
> Juergen
> 

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