Commit 120798d2 authored Oct 06, 2015 by Will Deacon Committed by Catalin Marinas Oct 07, 2015

arm64: mm: remove dsb from update_mmu_cache



update_mmu_cache() consists of a dsb(ishst) instruction so that new user
mappings are guaranteed to be visible to the page table walker on
exception return.

In reality this can be a very expensive operation which is rarely needed.
Removing this barrier shows a modest improvement in hackbench scores and
, in the worst case, we re-take the user fault and establish that there
was nothing to do.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

parent 28c6fbc3

arch/arm64/include/asm/pgtable.h

+3 −3

Original line number	Diff line number	Diff line
		@@ -646,10 +646,10 @@ static inline void update_mmu_cache(struct vm_area_struct *vma,
		unsigned long addr, pte_t *ptep)
		{
		/*
		* set_pte() does not have a DSB for user mappings, so make sure that
		* the page table write is visible.
		* We don't do anything here, so there's a very small chance of
		* us retaking a user fault which we just fixed up. The alternative
		* is doing a dsb(ishst), but that penalises the fastpath.
		*/
		dsb(ishst);
		}

		#define update_mmu_cache_pmd(vma, address, pmd) do { } while (0)