Re: [tip:x86/mm] x86/boot/32: Defer resyncing initial_page_table until per-cpu is set up

[Jan Kiszka]

On 2017-05-08 14:34, Jan Kiszka wrote:
> On 2017-05-08 13:21, Andy Lutomirski wrote:
>> On Mon, May 8, 2017 at 2:32 AM, Andy Shevchenko
>> <andy.shevchenko@xxxxxxxxx> wrote:
>>> On Mon, May 8, 2017 at 9:31 AM, Jan Kiszka <jan.kiszka@xxxxxxxxxxx> wrote:
>>>> On 2017-03-23 10:14, tip-bot for Andy Lutomirski wrote:
>>>>> The x86 smpboot trampoline expects initial_page_table to have the
>>>>> GDT mapped.  If the GDT ends up in a virtually mapped per-cpu page,
>>>>> then it won't be in the page tables at all until perc-pu areas are
>>>>> set up.  The result will be a triple fault the first time that the
>>>>> CPU attempts to access the GDT after LGDT loads the perc-pu GDT.
>>>>>
>>>>> This appears to be an old bug, but somehow the GDT fixmap rework
>>>>> is triggering it.  This seems to have something to do with the
>>>>> memory layout.
>>>
>>>> This breaks the boot on our Intel Quark platform (IOT2000, similar to
>>>> Galileo Gen2). Reverting it over master makes it work again. Any idea
>>>> what goes wrong? Let me know how I can help debugging this.
>>>
>>> JFYI: As of today linux-next when _kexec:ed_ works fine to me
>>>
>>> Perhaps I can test this later with direct boot from SD card.
>>>
>>
>> The most likely explanation is that there's some code that needs the
>> page table synced and runs before setup_per_cpu_areas().  The relevant
>> init code is:
>>
>>     setup_arch(&command_line);
>>     mm_init_cpumask(&init_mm);
>>     setup_command_line(command_line);
>>     setup_nr_cpu_ids();
>>     setup_per_cpu_areas();
>>
>> so I didn't move it very far.  It would be awesome if we could get a
>> backtrace when the failure happens, but it's likely to be a triple
>> fault.  Is this an EFI boot?  I bet the failure is in efi_init().
> 
> Yes, it's an EFI thing. Unfortunately, I didn't make
> earlycon/earlyprintk work yet.
> 
>>
>> Could you try reverting just the deletions in the patch?  I.e. try a
>> kernel with both the old and the new copies of the code I moved.
> 
> Let me try that later. I can also move the new code around to nail down
> the dependency.
> 

I found the reason: your patch is very discriminating! Not the whole
world is multicore yet. ;)

setup_per_cpu_areas() is taken from mm/percpu.c in case of !CONFIG_SMP.
So the new home for the resync is not even built.

Any suggestions how to refactor things instead?

Jan

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