Messages in this thread | | | From | Andy Lutomirski <> | Date | Mon, 8 May 2017 04:21:29 -0700 | Subject | Re: [tip:x86/mm] x86/boot/32: Defer resyncing initial_page_table until per-cpu is set up |
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On Mon, May 8, 2017 at 2:32 AM, Andy Shevchenko <andy.shevchenko@gmail.com> wrote: > On Mon, May 8, 2017 at 9:31 AM, Jan Kiszka <jan.kiszka@siemens.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
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