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Subject[PATCH v2] x86/head64: micro optimization: use memset instead of for loop
Date
This patch simplifies clearing of page table directories, by
the using of the fast memset() from the arch/x86/lib/memset_64.S.
Besides this, we 25 bytes is 25 bytes less than original.

text data bss dec hex filename

9735144 4970776 15474688 30180608 1cc8500 vmlinux.old
9735119 4970776 15474688 30180583 1cc84e7 vmlinux

Signed-off-by: Alexander Kuleshov <kuleshovmail@gmail.com>
---
Changelog:

v2: We got sizeof pud_p/pmd_p in the v1, but actually we need
to take sizeof of pud_p/pmd_p pointer, although their sizes are
the same.

arch/x86/kernel/head64.c | 14 +++-----------
1 file changed, 3 insertions(+), 11 deletions(-)

diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index 2c0f340..1f4422d 100644
--- a/arch/x86/kernel/head64.c
+++ b/arch/x86/kernel/head64.c
@@ -40,13 +40,8 @@ pmdval_t early_pmd_flags = __PAGE_KERNEL_LARGE & ~(_PAGE_GLOBAL | _PAGE_NX);
/* Wipe all early page tables except for the kernel symbol map */
static void __init reset_early_page_tables(void)
{
- unsigned long i;
-
- for (i = 0; i < PTRS_PER_PGD-1; i++)
- early_level4_pgt[i].pgd = 0;
-
+ memset(early_level4_pgt, 0, sizeof(pgd_t)*(PTRS_PER_PGD-1));
next_early_pgt = 0;
-
write_cr3(__pa_nodebug(early_level4_pgt));
}

@@ -54,7 +49,6 @@ static void __init reset_early_page_tables(void)
int __init early_make_pgtable(unsigned long address)
{
unsigned long physaddr = address - __PAGE_OFFSET;
- unsigned long i;
pgdval_t pgd, *pgd_p;
pudval_t pud, *pud_p;
pmdval_t pmd, *pmd_p;
@@ -81,8 +75,7 @@ again:
}

pud_p = (pudval_t *)early_dynamic_pgts[next_early_pgt++];
- for (i = 0; i < PTRS_PER_PUD; i++)
- pud_p[i] = 0;
+ memset(pud_p, 0, sizeof(*pud_p) * PTRS_PER_PUD);
*pgd_p = (pgdval_t)pud_p - __START_KERNEL_map + phys_base + _KERNPG_TABLE;
}
pud_p += pud_index(address);
@@ -97,8 +90,7 @@ again:
}

pmd_p = (pmdval_t *)early_dynamic_pgts[next_early_pgt++];
- for (i = 0; i < PTRS_PER_PMD; i++)
- pmd_p[i] = 0;
+ memset(pmd_p, 0, sizeof(*pmd_p) * PTRS_PER_PMD);
*pud_p = (pudval_t)pmd_p - __START_KERNEL_map + phys_base + _KERNPG_TABLE;
}
pmd = (physaddr & PMD_MASK) + early_pmd_flags;
--
2.7.0.25.gfc10eb5
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