[RFC][PATCH 4/9] arm64/arch_timer: Provide noinstr sched_clock_read() functions

[Peter Zijlstra <peterz@infradead.org>]

With the intent to provide local_clock_noinstr(), a variant of
local_clock() that's safe to be called from noinstr code (with the
assumption that any such code will already be non-preemptible),
prepare for things by providing a noinstr sched_clock_read() function.

Specifically, preempt_enable_*() calls out to schedule(), which upsets
noinstr validation efforts.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 arch/arm64/include/asm/arch_timer.h  |    8 ----
 drivers/clocksource/arm_arch_timer.c |   60 ++++++++++++++++++++++++++---------
 2 files changed, 47 insertions(+), 21 deletions(-)

--- a/arch/arm64/include/asm/arch_timer.h
+++ b/arch/arm64/include/asm/arch_timer.h
@@ -88,13 +88,7 @@ static inline notrace u64 arch_timer_rea
 
 #define arch_timer_reg_read_stable(reg)					\
 	({								\
-		u64 _val;						\
-									\
-		preempt_disable_notrace();				\
-		_val = erratum_handler(read_ ## reg)();			\
-		preempt_enable_notrace();				\
-									\
-		_val;							\
+		erratum_handler(read_ ## reg)();			\
 	})
 
 /*
--- a/drivers/clocksource/arm_arch_timer.c
+++ b/drivers/clocksource/arm_arch_timer.c
@@ -191,22 +191,40 @@ u32 arch_timer_reg_read(int access, enum
 	return val;
 }
 
-static notrace u64 arch_counter_get_cntpct_stable(void)
+static noinstr u64 _arch_counter_get_cntpct_stable(void)
 {
 	return __arch_counter_get_cntpct_stable();
 }
 
-static notrace u64 arch_counter_get_cntpct(void)
+static notrace u64 arch_counter_get_cntpct_stable(void)
+{
+	u64 val;
+	preempt_disable_notrace();
+	val = __arch_counter_get_cntpct_stable();
+	preempt_enable_notrace();
+	return val;
+}
+
+static noinstr u64 arch_counter_get_cntpct(void)
 {
 	return __arch_counter_get_cntpct();
 }
 
-static notrace u64 arch_counter_get_cntvct_stable(void)
+static noinstr u64 _arch_counter_get_cntvct_stable(void)
 {
 	return __arch_counter_get_cntvct_stable();
 }
 
-static notrace u64 arch_counter_get_cntvct(void)
+static notrace u64 arch_counter_get_cntvct_stable(void)
+{
+	u64 val;
+	preempt_disable_notrace();
+	val = __arch_counter_get_cntvct_stable();
+	preempt_enable_notrace();
+	return val;
+}
+
+static noinstr u64 arch_counter_get_cntvct(void)
 {
 	return __arch_counter_get_cntvct();
 }
@@ -753,14 +771,14 @@ static int arch_timer_set_next_event_phy
 	return 0;
 }
 
-static u64 arch_counter_get_cnt_mem(struct arch_timer *t, int offset_lo)
+static noinstr u64 arch_counter_get_cnt_mem(struct arch_timer *t, int offset_lo)
 {
 	u32 cnt_lo, cnt_hi, tmp_hi;
 
 	do {
-		cnt_hi = readl_relaxed(t->base + offset_lo + 4);
-		cnt_lo = readl_relaxed(t->base + offset_lo);
-		tmp_hi = readl_relaxed(t->base + offset_lo + 4);
+		cnt_hi = __raw_readl(t->base + offset_lo + 4);
+		cnt_lo = __raw_readl(t->base + offset_lo);
+		tmp_hi = __raw_readl(t->base + offset_lo + 4);
 	} while (cnt_hi != tmp_hi);
 
 	return ((u64) cnt_hi << 32) | cnt_lo;
@@ -1060,7 +1078,7 @@ bool arch_timer_evtstrm_available(void)
 	return cpumask_test_cpu(raw_smp_processor_id(), &evtstrm_available);
 }
 
-static u64 arch_counter_get_cntvct_mem(void)
+static noinstr u64 arch_counter_get_cntvct_mem(void)
 {
 	return arch_counter_get_cnt_mem(arch_timer_mem, CNTVCT_LO);
 }
@@ -1074,6 +1092,13 @@ struct arch_timer_kvm_info *arch_timer_g
 
 static void __init arch_counter_register(unsigned type)
 {
+	/*
+	 * Default to cp15 based access because arm64 uses this function for
+	 * sched_clock() before DT is probed and the cp15 method is guaranteed
+	 * to exist on arm64. arm doesn't use this before DT is probed so even
+	 * if we don't have the cp15 accessors we won't have a problem.
+	 */
+	u64 (*scr)(void) = arch_counter_get_cntvct;
 	u64 start_count;
 	int width;
 
@@ -1083,21 +1108,28 @@ static void __init arch_counter_register
 
 		if ((IS_ENABLED(CONFIG_ARM64) && !is_hyp_mode_available()) ||
 		    arch_timer_uses_ppi == ARCH_TIMER_VIRT_PPI) {
-			if (arch_timer_counter_has_wa())
+			if (arch_timer_counter_has_wa()) {
 				rd = arch_counter_get_cntvct_stable;
-			else
+				scr = _arch_counter_get_cntvct_stable;
+			} else {
 				rd = arch_counter_get_cntvct;
+				scr = arch_counter_get_cntvct;
+			}
 		} else {
-			if (arch_timer_counter_has_wa())
+			if (arch_timer_counter_has_wa()) {
 				rd = arch_counter_get_cntpct_stable;
-			else
+				scr = _arch_counter_get_cntpct_stable;
+			} else {
 				rd = arch_counter_get_cntpct;
+				scr = arch_counter_get_cntpct;
+			}
 		}
 
 		arch_timer_read_counter = rd;
 		clocksource_counter.vdso_clock_mode = vdso_default;
 	} else {
 		arch_timer_read_counter = arch_counter_get_cntvct_mem;
+		scr = arch_counter_get_cntvct_mem;
 	}
 
 	width = arch_counter_get_width();
@@ -1113,7 +1145,7 @@ static void __init arch_counter_register
 	timecounter_init(&arch_timer_kvm_info.timecounter,
 			 &cyclecounter, start_count);
 
-	sched_clock_register(arch_timer_read_counter, width, arch_timer_rate);
+	sched_clock_register(scr, width, arch_timer_rate);
 }
 
 static void arch_timer_stop(struct clock_event_device *clk)