Re: [Xen-devel] [PATCH RFC] VT-d: honor firmware-first mode in XSA-59 workaround code

["Zhang, Yang Z" <yang.z.zhang@xxxxxxxxx>]

Jan Beulich wrote on 2014-05-23:
>>>> On 23.05.14 at 03:03, <yang.z.zhang@xxxxxxxxx> wrote:
>> Andrew Cooper wrote on 2014-05-22:
>>> On 22/05/14 11:33, Jan Beulich wrote:
>>>>>>> On 22.05.14 at 12:19, <andrew.cooper3@xxxxxxxxxx> wrote:
>>>>> No.  We have not observed an issue from XSA-59.
>>>>> 
>>>>> The version of XenServer we had the issue with didn't contain any
>>>>> of the
>>>>> XSA-59 fixes at the point that the problem was observed.
>>>> Then what was yesterday's alert about then? I.e. do we have any
>>>> indication that the workaround as is may cause problems, and that
>>>> hence the (relatively involved) patch here is needed at all? And,
>>>> how are you intending to test this patch if you haven't even seen
>>>> an issue?
>>> 
>>> As part of finding the root cause of our issue, we identified that
>>> just as Dom0 must not play with AER in firmware first mode, Xen
>>> must not
>> play either.
>> 
>> I saw upstream Linux has the patch to handle this case in 2009.
> 
> Yes, yet all patches so far went through without anyone (me
> included) pointing this out during review.
> 
>>> I believe that we have XSA-59 affected hardware with both
>>> firmware-first and non-firmware-first HEST tables, so we should be
>> 
>> why non-firmware-first hardware also affected? It seems only
>> firmware-first hardware is buggy.
> 
> "Buggy" is perhaps the wrong term here; "problematic" would seem a
> better fit. And I think you misread Andrew's reply - we certainly need
> to test the patch on both HEST-with-FF and HEST-without-FF systems,
> even if only the former would suffer from the presumed regression.
> 

Yes, we definitely need to test it on both machines. From all your previous 
discussions, the cause is that XSA-59 touches AER register without consider 
whether it is owned by firmware. But if hardware doesn't support 
firmware-first, then it has no problem.

Best regards,
Yang



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