ARM: Remove __ARCH_WANT_INTERRUPTS_ON_CTXSW on pre-ARMv6 CPUs

This patch removes the __ARCH_WANT_INTERRUPTS_ON_CTXSW definition for
ARMv5 and earlier processors. On such processors, the context switch
requires a full cache flush. To avoid high interrupt latencies, this
patch defers the mm switching to the post-lock switch hook if the
interrupts are disabled.
Reviewed-by: NWill Deacon <will.deacon@arm.com>
Tested-by: NWill Deacon <will.deacon@arm.com>
Reviewed-by: NFrank Rowand <frank.rowand@am.sony.com>
Tested-by: NMarc Zyngier <Marc.Zyngier@arm.com>
Signed-off-by: NCatalin Marinas <catalin.marinas@arm.com>

arch/arm/include/asm/mmu.h

@@ -34,13 +34,4 @@ typedef struct {
 
 #endif
 
-/*
- * switch_mm() may do a full cache flush over the context switch,
- * so enable interrupts over the context switch to avoid high
- * latency.
- */
-#ifndef CONFIG_CPU_HAS_ASID
-#define __ARCH_WANT_INTERRUPTS_ON_CTXSW
-#endif
-
 #endif

arch/arm/include/asm/mmu_context.h

@@ -105,19 +105,40 @@ static inline void finish_arch_post_lock_switch(void)
 
 #else	/* !CONFIG_CPU_HAS_ASID */
 
+#ifdef CONFIG_MMU
+
 static inline void check_and_switch_context(struct mm_struct *mm,
 					    struct task_struct *tsk)
 {
-#ifdef CONFIG_MMU
 	if (unlikely(mm->context.kvm_seq != init_mm.context.kvm_seq))
 		__check_kvm_seq(mm);
-	cpu_switch_mm(mm->pgd, mm);
-#endif
+
+	if (irqs_disabled())
+		/*
+		 * cpu_switch_mm() needs to flush the VIVT caches. To avoid
+		 * high interrupt latencies, defer the call and continue
+		 * running with the old mm. Since we only support UP systems
+		 * on non-ASID CPUs, the old mm will remain valid until the
+		 * finish_arch_post_lock_switch() call.
+		 */
+		set_ti_thread_flag(task_thread_info(tsk), TIF_SWITCH_MM);
+	else
+		cpu_switch_mm(mm->pgd, mm);
 }
 
-#define init_new_context(tsk,mm)	0
+#define finish_arch_post_lock_switch \
+	finish_arch_post_lock_switch
+static inline void finish_arch_post_lock_switch(void)
+{
+	if (test_and_clear_thread_flag(TIF_SWITCH_MM)) {
+		struct mm_struct *mm = current->mm;
+		cpu_switch_mm(mm->pgd, mm);
+	}
+}
 
-#define finish_arch_post_lock_switch()	do { } while (0)
+#endif	/* CONFIG_MMU */
+
+#define init_new_context(tsk,mm)	0
 
 #endif	/* CONFIG_CPU_HAS_ASID */