Re: [Xen-devel] crash on boot with 4.6.1 on fedora 24

[Juergen Gross <jgross@xxxxxxxx>]

On 11/05/16 08:35, Jan Beulich wrote:
>>>> On 11.05.16 at 07:49, <JGross@xxxxxxxx> wrote:
>> On 10/05/16 18:35, Boris Ostrovsky wrote:
>>> On 05/10/2016 11:43 AM, Juergen Gross wrote:
>>>> On 10/05/16 17:35, Jan Beulich wrote:
>>>>>>>> On 10.05.16 at 17:19, <JGross@xxxxxxxx> wrote:
>>>>>> On 10/05/16 15:57, Jan Beulich wrote:
>>>>>>>>>> On 10.05.16 at 15:39, <boris.ostrovsky@xxxxxxxxxx> wrote:
>>>>>>>> I didn't finish unwrapping the stack yesterday. Here it is:
>>>>>>>>
>>>>>>>> setup_arch -> dmi_scan_machine -> dmi_walk_early -> early_ioremap
>>>>>>> Ah, that makes sense. Yet why would early_ioremap() involve an
>>>>>>> M2P lookup? As said, MMIO addresses shouldn't be subject to such
>>>>>>> lookups.
>>>>>> early_ioremap()->
>>>>>>   __early_ioremap()->
>>>>>>     __early_set_fixmap()->
>>>>>>       set_pte()->
>>>>>>         xen_set_pte_init()->
>>>>>>           mask_rw_pte()->
>>>>>>             pte_pfn()->
>>>>>>               pte_val()->
>>>>>>                 xen_pte_val()->
>>>>>>                   pte_mfn_to_pfn()
>>>>> Well, I understand (also from Boris' first reply) that's how it is,
>>>>> but not why it is so. I.e. the call flow above doesn't answer my
>>>>> question.
>>>> On x86 early_ioremap() and early_memremap() share a common sub-function
>>>> __early_ioremap(). This together with pvops requires a common set_pte()
>>>> implementation leading to the mfn validation in the end.
>>>
>>> Do we make any assumptions about where DMI data lives?
>>
>> I don't think so.
>>
>> So the basic problem is the page fault due to the sparse m2p map before
>> the #PF handler is registered.
>>
>> What do you think about registering a minimal #PF handler in
>> xen_arch_setup() being capable to handle this problem? This should be
>> doable without major problems. I can do a patch.
> 
> To me that would feel like working around the issue instead of
> admitting that the removal of _PAGE_IOMAP was a mistake.

Hmm, I don't think so.

Having a Xen specific pte flag seems to be much more intrusive than
having an early boot page fault handler consisting of just one line
being capable to mimic the default handler in just one aspect (see
attached patch - only compile tested).

Adding David as he removed _PAGE_IOMAP in kernel 3.18.


Juergen

Attachment: early-pf.patch
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