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Subject[PATCH v3 4/6] ARM: Remove __ARCH_WANT_INTERRUPTS_ON_CTXSW on ASID-capable CPUs
Date
Since the ASIDs must be unique to an mm across all the CPUs in a system,
the __new_context() function needs to broadcast a context reset event to
all the CPUs during ASID allocation if a roll-over occurred. Such IPIs
cannot be issued with interrupts disabled and ARM had to define
__ARCH_WANT_INTERRUPTS_ON_CTXSW.

This patch changes the check_context() function to
check_and_switch_context() called from switch_mm(). In case of
ASID-capable CPUs (ARMv6 onwards), if a new ASID is needed and the
interrupts are disabled, it defers the __new_context() and
cpu_switch_mm() calls to the post-lock switch hook where the interrupts
are enabled. Setting the reserved TTBR0 was also moved to
check_and_switch_context() from cpu_v7_switch_mm().

Reviewed-by: Will Deacon <will.deacon@arm.com>
Reviewed-by: Frank Rowand <frank.rowand@am.sony.com>
Cc: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
---
arch/arm/include/asm/mmu_context.h | 72 ++++++++++++++++++++++++++++--------
arch/arm/include/asm/system.h | 2 +
arch/arm/include/asm/thread_info.h | 1 +
arch/arm/mm/context.c | 4 +-
arch/arm/mm/proc-v7-2level.S | 3 -
5 files changed, 61 insertions(+), 21 deletions(-)

diff --git a/arch/arm/include/asm/mmu_context.h b/arch/arm/include/asm/mmu_context.h
index 71605d9..35483a8 100644
--- a/arch/arm/include/asm/mmu_context.h
+++ b/arch/arm/include/asm/mmu_context.h
@@ -48,39 +48,80 @@ DECLARE_PER_CPU(struct mm_struct *, current_mm);

void __init_new_context(struct task_struct *tsk, struct mm_struct *mm);
void __new_context(struct mm_struct *mm);
+void cpu_set_reserved_ttbr0(void);

-static inline void check_context(struct mm_struct *mm)
+static void switch_new_context(struct mm_struct *mm)
{
- /*
- * This code is executed with interrupts enabled. Therefore,
- * mm->context.id cannot be updated to the latest ASID version
- * on a different CPU (and condition below not triggered)
- * without first getting an IPI to reset the context. The
- * alternative is to take a read_lock on mm->context.id_lock
- * (after changing its type to rwlock_t).
- */
- if (unlikely((mm->context.id ^ cpu_last_asid) >> ASID_BITS))
- __new_context(mm);
+ unsigned long flags;

+ __new_context(mm);
+
+ local_irq_save(flags);
+ cpu_switch_mm(mm->pgd, mm);
+ local_irq_restore(flags);
+}
+
+static inline void check_and_switch_context(struct mm_struct *mm,
+ struct task_struct *tsk)
+{
if (unlikely(mm->context.kvm_seq != init_mm.context.kvm_seq))
__check_kvm_seq(mm);
+
+ /*
+ * Required during context switch to avoid speculative page table
+ * walking with the wrong TTBR.
+ */
+ cpu_set_reserved_ttbr0();
+
+ if (!((mm->context.id ^ cpu_last_asid) >> ASID_BITS))
+ /*
+ * The ASID is from the current generation, just switch to the
+ * new pgd. This condition is only true for calls from
+ * context_switch() and interrupts are already disabled.
+ */
+ cpu_switch_mm(mm->pgd, mm);
+ else if (irqs_disabled())
+ /*
+ * Defer the new ASID allocation until after the context
+ * switch critical region since __new_context() cannot be
+ * called with interrupts disabled (it sends IPIs).
+ */
+ set_ti_thread_flag(task_thread_info(tsk), TIF_SWITCH_MM);
+ else
+ /*
+ * That is a direct call to switch_mm() or activate_mm() with
+ * interrupts enabled and a new context.
+ */
+ switch_new_context(mm);
}

#define init_new_context(tsk,mm) (__init_new_context(tsk,mm),0)

-#else
+#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))
+ switch_new_context(current->mm);
+}

-static inline void check_context(struct mm_struct *mm)
+#else /* !CONFIG_CPU_HAS_ASID */
+
+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
}

#define init_new_context(tsk,mm) 0

-#endif
+#define finish_arch_post_lock_switch() do { } while (0)
+
+#endif /* CONFIG_CPU_HAS_ASID */

#define destroy_context(mm) do { } while(0)

@@ -122,8 +163,7 @@ switch_mm(struct mm_struct *prev, struct mm_struct *next,
struct mm_struct **crt_mm = &per_cpu(current_mm, cpu);
*crt_mm = next;
#endif
- check_context(next);
- cpu_switch_mm(next->pgd, next);
+ check_and_switch_context(next, tsk);
if (cache_is_vivt())
cpumask_clear_cpu(cpu, mm_cpumask(prev));
}
diff --git a/arch/arm/include/asm/system.h b/arch/arm/include/asm/system.h
index e4c96cc..e4b41dd 100644
--- a/arch/arm/include/asm/system.h
+++ b/arch/arm/include/asm/system.h
@@ -230,7 +230,9 @@ static inline void set_copro_access(unsigned int val)
* so enable interrupts over the context switch to avoid high
* latency.
*/
+#ifndef CONFIG_CPU_HAS_ASID
#define __ARCH_WANT_INTERRUPTS_ON_CTXSW
+#endif

/*
* switch_to(prev, next) should switch from task `prev' to `next'
diff --git a/arch/arm/include/asm/thread_info.h b/arch/arm/include/asm/thread_info.h
index d4c24d4..9e13e33 100644
--- a/arch/arm/include/asm/thread_info.h
+++ b/arch/arm/include/asm/thread_info.h
@@ -146,6 +146,7 @@ extern void vfp_flush_hwstate(struct thread_info *);
#define TIF_MEMDIE 18 /* is terminating due to OOM killer */
#define TIF_RESTORE_SIGMASK 20
#define TIF_SECCOMP 21
+#define TIF_SWITCH_MM 22 /* deferred switch_mm */

#define _TIF_SIGPENDING (1 << TIF_SIGPENDING)
#define _TIF_NEED_RESCHED (1 << TIF_NEED_RESCHED)
diff --git a/arch/arm/mm/context.c b/arch/arm/mm/context.c
index efa413a..ab35a86 100644
--- a/arch/arm/mm/context.c
+++ b/arch/arm/mm/context.c
@@ -23,7 +23,7 @@ DEFINE_PER_CPU(struct mm_struct *, current_mm);
#endif

#ifdef CONFIG_ARM_LPAE
-static void cpu_set_reserved_ttbr0(void)
+void cpu_set_reserved_ttbr0(void)
{
unsigned long ttbl = __pa(swapper_pg_dir);
unsigned long ttbh = 0;
@@ -39,7 +39,7 @@ static void cpu_set_reserved_ttbr0(void)
isb();
}
#else
-static void cpu_set_reserved_ttbr0(void)
+void cpu_set_reserved_ttbr0(void)
{
u32 ttb;
/* Copy TTBR1 into TTBR0 */
diff --git a/arch/arm/mm/proc-v7-2level.S b/arch/arm/mm/proc-v7-2level.S
index 7227048..42ac069 100644
--- a/arch/arm/mm/proc-v7-2level.S
+++ b/arch/arm/mm/proc-v7-2level.S
@@ -46,9 +46,6 @@ ENTRY(cpu_v7_switch_mm)
#ifdef CONFIG_ARM_ERRATA_430973
mcr p15, 0, r2, c7, c5, 6 @ flush BTAC/BTB
#endif
- mrc p15, 0, r2, c2, c0, 1 @ load TTB 1
- mcr p15, 0, r2, c2, c0, 0 @ into TTB 0
- isb
#ifdef CONFIG_ARM_ERRATA_754322
dsb
#endif


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