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

[Juergen Gross <jgross@xxxxxxxx>]

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.


Juergen

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