Re: [PATCH v3 04/21] arm64: decouple early fixmap init from linear mapping

[Ard Biesheuvel]

On 11 January 2016 at 17:51, Mark Rutland <mark.rutland@xxxxxxx> wrote:
> On Mon, Jan 11, 2016 at 04:27:38PM +0000, Mark Rutland wrote:
>> On Mon, Jan 11, 2016 at 05:15:13PM +0100, Ard Biesheuvel wrote:
>> > On 11 January 2016 at 17:09, Mark Rutland <mark.rutland@xxxxxxx> wrote:
>> > > On Mon, Jan 11, 2016 at 02:18:57PM +0100, 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.
>> > >>
>> > >> Since the fixmap page tables are disjoint from the kernel mapping up
>> > >> to the top level pgd entry, we can refer to bm_pte[] directly, and there
>> > >> is no need to walk the page tables and perform __pa()/__va() translations
>> > >> at each step.
>> > >>
>> > >> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
>> > >> ---
>> > >>  arch/arm64/mm/mmu.c | 32 ++++++--------------
>> > >>  1 file changed, 9 insertions(+), 23 deletions(-)
>> > >>
>> > >> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>> > >> index 7711554a94f4..75b5f0dc3bdc 100644
>> > >> --- a/arch/arm64/mm/mmu.c
>> > >> +++ b/arch/arm64/mm/mmu.c
>> > >> @@ -570,38 +570,24 @@ void vmemmap_free(unsigned long start, unsigned long end)
>> > >>  #endif       /* CONFIG_SPARSEMEM_VMEMMAP */
>> > >>
>> > >>  static pte_t bm_pte[PTRS_PER_PTE] __page_aligned_bss;
>> > >> -#if CONFIG_PGTABLE_LEVELS > 2
>> > >>  static pmd_t bm_pmd[PTRS_PER_PMD] __page_aligned_bss;
>> > >> -#endif
>> > >> -#if CONFIG_PGTABLE_LEVELS > 3
>> > >>  static pud_t bm_pud[PTRS_PER_PUD] __page_aligned_bss;
>> > >> -#endif
>> > >>
>> > >>  static inline pud_t * fixmap_pud(unsigned long addr)
>> > >>  {
>> > >> -     pgd_t *pgd = pgd_offset_k(addr);
>> > >> -
>> > >> -     BUG_ON(pgd_none(*pgd) || pgd_bad(*pgd));
>> > >> -
>> > >> -     return pud_offset(pgd, addr);
>> > >> +     return (CONFIG_PGTABLE_LEVELS > 3) ? &bm_pud[pud_index(addr)]
>> > >> +                                        : (pud_t *)pgd_offset_k(addr);
>> > >
>> > > If we move patch 6 earlier, we could use pud_offset_kimg here, and avoid
>> > > the cast, at the cost of passing the pgd into fixmap_pud.
>> > >
>> > > Similarly for fixmap_pmd.
>> > >
>> >
>> > Is that necessarily an improvement? I know it hides the cast, but I
>> > think having an explicit pgd_t* to pud_t* cast that so obviously
>> > applies to CONFIG_PGTABLE_LEVELS < 4 only is fine as well.
>>
>> True; it's not a big thing either way.
>
> Sorry,  I'm gonig to change my mind on that again. I think using
> p?d_offset_kimg is preferable. e.g.
>
> static inline pud_t * fixmap_pud(unsigned long addr)
> {
>         pgd_t *pgd = pgd_offset_k(addr);
>
>         BUG_ON(pgd_none(*pgd) || pgd_bad(*pgd));
>
>         return pud_offset_kimg(pgd, addr);
> }
>
> static inline pmd_t * fixmap_pmd(unsigned long addr)
> {
>         pud_t *pud = fixmap_pud(addr);
>
>         BUG_ON(pud_none(*pud) || pud_bad(*pud));
>
>         return pmd_offset_kimg(pud, addr);
> }
>
> That avoids having to check CONFIG_PGTABLE_LEVELS check and perform a cast,
> avoids duplicating details about bm_{pud,pmd}, and keeps the existing structure
> so it's easier to reason about the change. I was wrong about having to pass the
> pgd or pud in, so callers don't need upating.
>
> From my PoV that is preferable.
>

OK. I think it looks better, indeed.