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Commit c2775b2e authored by Will Deacon's avatar Will Deacon Committed by Catalin Marinas
Browse files

arm64: switch_mm: simplify mm and CPU checks 



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: default avatarCatalin Marinas <catalin.marinas@arm.com>
Signed-off-by: default avatarWill Deacon <will.deacon@arm.com>
Signed-off-by: default avatarCatalin Marinas <catalin.marinas@arm.com>
parent 5a7862e8

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);

}