linux-kernel - Re: [tip:x86/mm] x86/boot/32: Defer resyncing initial_page

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Date:   Mon, 8 May 2017 04:21:29 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Jan Kiszka <jan.kiszka@...mens.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...nel.org>, x86 <x86@...nel.org>,
        linux-efi <linux-efi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        Thomas Gleixner <tglx@...utronix.de>,
        Brian Gerst <brgerst@...il.com>,
        Thomas Garnier <thgarnie@...gle.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Juergen Gross <jgross@...e.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>
Subject: Re: [tip:x86/mm] x86/boot/32: Defer resyncing initial_page_table
 until per-cpu is set up

On Mon, May 8, 2017 at 2:32 AM, Andy Shevchenko
<andy.shevchenko@...il.com> wrote:
> On Mon, May 8, 2017 at 9:31 AM, Jan Kiszka <jan.kiszka@...mens.com> 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().

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.

--Andy