Re: [PATCH v2 04/13] arm64: decouple early fixmap init from linear mapping

Ard Biesheuvel
2016-01-06T16:42:59Z

On 6 January 2016 at 17:35, James Morse <james.morse@arm.com> wrote:
> Hi Ard!
>
> On 30/12/15 15:26, Ard Biesheuvel wrote:
>> Since the early fixmap page tables are populated using pages that are
>> part of the static footprint of the kernel, they are covered by the
>> initial kernel mapping, and we can refer to them without using __va/__pa
>> translations, which are tied to the linear mapping.
>>
>> Instead, let's introduce __phys_to_kimg, which will be tied to the kernel
>> virtual mapping, regardless of whether or not it intersects with the linear
>> mapping. This will allow us to move the kernel out of the linear mapping in
>> a subsequent patch.
>>
>
> I gave your arm64-kaslr-v2 branch a go on juno r1, currently with
> ARM64_RELOCATABLE_KERNEL=n, to find it didn't boot.
>
> git bisect pointed to this patch. From the debugger it looks like
> rubbish is ending up the page tables after early_fixmap_init(), printing
> bits of bm_pmd and friends shows these aren't zeroed.
>
> I think this is because the section(".pgdir") is dragging these outside
> the __bss_start/__bss_stop range that is zeroed in head.S:__mmap_switched().
>

Thanks for spotting that! This code runs happily on my Seattle A0, but
it is obviously incorrect.

> The following inelegant patch fixes this problem for me:
> ----------------------------%<----------------------------
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index a78fc5a882da..15fc9712ddc1 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -559,6 +559,7 @@ void __init early_fixmap_init(void)
> if (pgd_none(*pgd)) {
> static pud_t bm_pud[PTRS_PER_PUD] __pgdir;
>
> + memset(bm_pud, 0, sizeof(bm_pud));
> pgd_populate(&init_mm, pgd, bm_pud);
> memblock_reserve(__pa(bm_pud), sizeof(bm_pud));
> }
> @@ -570,6 +571,7 @@ void __init early_fixmap_init(void)
> if (pud_none(*pud)) {
> static pmd_t bm_pmd[PTRS_PER_PMD] __pgdir;
>
> + memset(bm_pmd, 0, sizeof(bm_pmd));
> pud_populate(&init_mm, pud, bm_pmd);
> memblock_reserve(__pa(bm_pmd), sizeof(bm_pmd));
> }
> @@ -580,6 +582,7 @@ void __init early_fixmap_init(void)
> if (pmd_none(*pmd)) {
> static pte_t bm_pte[PTRS_PER_PTE] __pgdir;
>
> + memset(bm_pte, 0, sizeof(bm_pte));
> pmd_populate_kernel(&init_mm, pmd, bm_pte);
> memblock_reserve(__pa(bm_pte), sizeof(bm_pte));
> }
> ----------------------------%<----------------------------
>

Actually, this looks fine to me. I will fold this into my patch

NOTE: I have a -v3 version up on git.linaro.org now, with a couple of changes.

Thanks!

Ard.