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:

quoted

On 11 January 2016 at 17:09, Mark Rutland [off-list ref] wrote:

quoted

On Mon, Jan 11, 2016 at 02:18:57PM +0100, Ard Biesheuvel wrote:

quoted

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 <redacted>
---
 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.