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

[Jan Kiszka]

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.

Jan

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