Messages in this thread Patch in this message | | | Date | Mon, 11 Jan 2016 16:51:04 +0000 | From | Mark Rutland <> | Subject | Re: [PATCH v3 04/21] arm64: decouple early fixmap init from linear mapping |
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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@arm.com> 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@linaro.org> > > >> --- > > >> 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.
Thanks, Mark.
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