arm64: switch_mm: simplify mm and CPU checks (c2775b2e) · Commits · 戴 / test

switch_mm performs some checks to try and avoid entering the ASID allocator: (1) If we're switching to the init_mm (no user mappings), then simply set a reserved TTBR0 value with no page table (the zero page) (2) If prev == next *and* the mm_cpumask indicates that we've run on this CPU before, then we can skip the allocator. However, there is plenty of redundancy here. With the new ASID allocator, if prev == next, then we know that our ASID is valid and do not need to worry about re-allocation. Consequently, we can drop the mm_cpumask check in (2) and move the prev == next check before the init_mm check, since if prev == next == init_mm then there's 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>

arch/arm64/include/asm/mmu_context.h

+4 −2

Original line number	Diff line number	Diff line
		@@ -129,6 +129,9 @@ switch_mm(struct mm_struct prev, struct mm_struct next,
		{
		unsigned int cpu = smp_processor_id();

		if (prev == next)
		return;

		/*
		* init_mm.pgd does not contain any user mappings and it is always
		* active for kernel addresses in TTBR1. Just set the reserved TTBR0.
		@@ -138,8 +141,7 @@ switch_mm(struct mm_struct prev, struct mm_struct next,
		return;
		}

		if (!cpumask_test_and_set_cpu(cpu, mm_cpumask(next)) \|\| prev != next)
		check_and_switch_context(next, tsk);
		check_and_switch_context(next, cpu);
		}

		#define deactivate_mm(tsk,mm) do { } while (0)

arch/arm64/mm/context.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -166,10 +166,10 @@ void check_and_switch_context(struct mm_struct *mm, unsigned int cpu)
		local_flush_tlb_all();

		atomic64_set(&per_cpu(active_asids, cpu), asid);
		cpumask_set_cpu(cpu, mm_cpumask(mm));
		raw_spin_unlock_irqrestore(&cpu_asid_lock, flags);

		switch_mm_fastpath:
		cpumask_set_cpu(cpu, mm_cpumask(mm));
		cpu_switch_mm(mm->pgd, mm);
		}

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