arm64: mm: rewrite ASID allocator and MM context-switching code

Our current switch_mm implementation suffers from a number of problems:

  (1) The ASID allocator relies on IPIs to synchronise the CPUs on a
      rollover event

  (2) Because of (1), we cannot allocate ASIDs with interrupts disabled
      and therefore make use of a TIF_SWITCH_MM flag to postpone the
      actual switch to finish_arch_post_lock_switch

  (3) We run context switch with a reserved (invalid) TTBR0 value, even
      though the ASID and pgd are updated atomically

  (4) We take a global spinlock (cpu_asid_lock) during context-switch

  (5) We use h/w broadcast TLB operations when they are not required
      (e.g. in flush_context)

This patch addresses these problems by rewriting the ASID algorithm to
match the bitmap-based arch/arm/ implementation more closely. This in
turn allows us to remove much of the complications surrounding switch_mm,
including the ugly thread flag.

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>

1 files changed, 8 insertions, 7 deletions

diff --git a/arch/arm64/include/asm/mmu.h b/arch/arm64/include/asm/mmu.h
index 030208767185..990124a67eeb 100644
--- a/arch/arm64/include/asm/mmu.h
+++ b/arch/arm64/include/asm/mmu.h
@@ -17,15 +17,16 @@
 #define __ASM_MMU_H
 
 typedef struct {
-	unsigned int id;
-	raw_spinlock_t id_lock;
-	void *vdso;
+	atomic64_t	id;
+	void		*vdso;
 } mm_context_t;
 
-#define INIT_MM_CONTEXT(name) \
-	.context.id_lock = __RAW_SPIN_LOCK_UNLOCKED(name.context.id_lock),
-
-#define ASID(mm)	((mm)->context.id & 0xffff)
+/*
+ * This macro is only used by the TLBI code, which cannot race with an
+ * ASID change and therefore doesn't need to reload the counter using
+ * atomic64_read.
+ */
+#define ASID(mm)	((mm)->context.id.counter & 0xffff)
 
 extern void paging_init(void);
 extern void __iomem *early_io_map(phys_addr_t phys, unsigned long virt);