MIPS: HTW: Prevent accidental HTW start due to nested htw_{start, stop}

activate_mm() and switch_mm() call get_new_mmu_context() which in turn
can enable the HTW before the entryhi is changed with the new ASID.
Since the latter will enable the HTW in local_flush_tlb_all(),
then there is a small timing window where the HTW is running with the
new ASID but with an old pgd since the TLBMISS_HANDLER_SETUP_PGD
hasn't assigned a new one yet. In order to prevent that, we introduce a
simple htw counter to avoid starting HTW accidentally due to nested
htw_{start,stop}() sequences. Moreover, since various IPI calls can
enforce TLB flushing operations on a different core, such an operation
may interrupt another htw_{stop,start} in progress leading inconsistent
updates of the htw_seq variable. In order to avoid that, we disable the
interrupts whenever we update that variable.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Cc: <stable@vger.kernel.org> # 3.17+
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/9118/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/include/asm/cpu-info.h

@@ -84,6 +84,11 @@ struct cpuinfo_mips {
 	 * (shifted by _CACHE_SHIFT)
 	 */
 	unsigned int		writecombine;
+	/*
+	 * Simple counter to prevent enabling HTW in nested
+	 * htw_start/htw_stop calls
+	 */
+	unsigned int		htw_seq;
 } __attribute__((aligned(SMP_CACHE_BYTES)));
 
 extern struct cpuinfo_mips cpu_data[];

arch/mips/include/asm/mmu_context.h

@@ -25,7 +25,6 @@ do {									\
 	if (cpu_has_htw) {						\
 		write_c0_pwbase(pgd);					\
 		back_to_back_c0_hazard();				\
-		htw_reset();						\
 	}								\
 } while (0)
 
@@ -144,6 +143,7 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
 	unsigned long flags;
 	local_irq_save(flags);
 
+	htw_stop();
 	/* Check if our ASID is of an older version and thus invalid */
 	if ((cpu_context(cpu, next) ^ asid_cache(cpu)) & ASID_VERSION_MASK)
 		get_new_mmu_context(next, cpu);
@@ -156,6 +156,7 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
 	 */
 	cpumask_clear_cpu(cpu, mm_cpumask(prev));
 	cpumask_set_cpu(cpu, mm_cpumask(next));
+	htw_start();
 
 	local_irq_restore(flags);
 }
@@ -182,6 +183,7 @@ activate_mm(struct mm_struct *prev, struct mm_struct *next)
 
 	local_irq_save(flags);
 
+	htw_stop();
 	/* Unconditionally get a new ASID.  */
 	get_new_mmu_context(next, cpu);
 
@@ -191,6 +193,7 @@ activate_mm(struct mm_struct *prev, struct mm_struct *next)
 	/* mark mmu ownership change */
 	cpumask_clear_cpu(cpu, mm_cpumask(prev));
 	cpumask_set_cpu(cpu, mm_cpumask(next));
+	htw_start();
 
 	local_irq_restore(flags);
 }
@@ -205,6 +208,7 @@ drop_mmu_context(struct mm_struct *mm, unsigned cpu)
 	unsigned long flags;
 
 	local_irq_save(flags);
+	htw_stop();
 
 	if (cpumask_test_cpu(cpu, mm_cpumask(mm)))  {
 		get_new_mmu_context(mm, cpu);
@@ -213,6 +217,7 @@ drop_mmu_context(struct mm_struct *mm, unsigned cpu)
 		/* will get a new context next time */
 		cpu_context(cpu, mm) = 0;
 	}
+	htw_start();
 	local_irq_restore(flags);
 }
 

arch/mips/include/asm/pgtable.h

@@ -99,19 +99,31 @@ extern void paging_init(void);
 
 #define htw_stop()							\
 do {									\
+	unsigned long flags;						\
+									\
 	if (cpu_has_htw) {						\
-		write_c0_pwctl(read_c0_pwctl() &			\
-			       ~(1 << MIPS_PWCTL_PWEN_SHIFT));		\
-		back_to_back_c0_hazard();				\
+		local_irq_save(flags);					\
+		if(!raw_current_cpu_data.htw_seq++) {			\
+			write_c0_pwctl(read_c0_pwctl() &		\
+				       ~(1 << MIPS_PWCTL_PWEN_SHIFT));	\
+			back_to_back_c0_hazard();			\
+		}							\
+		local_irq_restore(flags);				\
 	}								\
 } while(0)
 
 #define htw_start()							\
 do {									\
+	unsigned long flags;						\
+									\
 	if (cpu_has_htw) {						\
-		write_c0_pwctl(read_c0_pwctl() |			\
-			       (1 << MIPS_PWCTL_PWEN_SHIFT));		\
-		back_to_back_c0_hazard();				\
+		local_irq_save(flags);					\
+		if (!--raw_current_cpu_data.htw_seq) {			\
+			write_c0_pwctl(read_c0_pwctl() |		\
+				       (1 << MIPS_PWCTL_PWEN_SHIFT));	\
+			back_to_back_c0_hazard();			\
+		}							\
+		local_irq_restore(flags);				\
 	}								\
 } while(0)
 

arch/mips/kernel/cpu-probe.c

@@ -424,8 +424,10 @@ static inline unsigned int decode_config3(struct cpuinfo_mips *c)
 	if (config3 & MIPS_CONF3_MSA)
 		c->ases |= MIPS_ASE_MSA;
 	/* Only tested on 32-bit cores */
-	if ((config3 & MIPS_CONF3_PW) && config_enabled(CONFIG_32BIT))
+	if ((config3 & MIPS_CONF3_PW) && config_enabled(CONFIG_32BIT)) {
+		c->htw_seq = 0;
 		c->options |= MIPS_CPU_HTW;
+	}
 
 	return config3 & MIPS_CONF_M;
 }