[PATCH 15/17] x86: Add rdtsc_ordered() and use it in trivial call sites

[Andy Lutomirski]

barrier_before_rdtsc(); rdtsc_unordered() is an unnecessary mouthful and
requires more thought than should be necessary.  Add an rdtsc_ordered()
helper and replace the trivial call sites with it.

This should not change generated code.

Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
---
 arch/x86/entry/vdso/vclock_gettime.c | 16 ++--------------
 arch/x86/include/asm/msr.h           | 14 ++++++++++++++
 arch/x86/kernel/trace_clock.c        |  7 +------
 arch/x86/kvm/x86.c                   | 16 ++--------------
 arch/x86/lib/delay.c                 |  9 +++------
 5 files changed, 22 insertions(+), 40 deletions(-)

diff --git a/arch/x86/entry/vdso/vclock_gettime.c b/arch/x86/entry/vdso/vclock_gettime.c
index 0f91cc7373df..ca94fa649251 100644
--- a/arch/x86/entry/vdso/vclock_gettime.c
+++ b/arch/x86/entry/vdso/vclock_gettime.c
@@ -175,20 +175,8 @@ static notrace cycle_t vread_pvclock(int *mode)
 
 notrace static cycle_t vread_tsc(void)
 {
-	cycle_t ret;
-	u64 last;
-
-	/*
-	 * Empirically, a fence (of type that depends on the CPU)
-	 * before rdtsc is enough to ensure that rdtsc is ordered
-	 * with respect to loads.  The various CPU manuals are unclear
-	 * as to whether rdtsc can be reordered with later loads,
-	 * but no one has ever seen it happen.
-	 */
-	barrier_before_rdtsc();
-	ret = (cycle_t)rdtsc_unordered();
-
-	last = gtod->cycle_last;
+	cycle_t ret = (cycle_t)rdtsc_ordered();
+	u64 last = gtod->cycle_last;
 
 	if (likely(ret >= last))
 		return ret;
diff --git a/arch/x86/include/asm/msr.h b/arch/x86/include/asm/msr.h
index a47fb11af5f5..22d69d2d1f0d 100644
--- a/arch/x86/include/asm/msr.h
+++ b/arch/x86/include/asm/msr.h
@@ -148,6 +148,20 @@ static __always_inline void barrier_before_rdtsc(void)
 			  "lfence", X86_FEATURE_LFENCE_RDTSC);
 }
 
+/**
+ * rdtsc_ordered() - read the current TSC in program order
+ *
+ * rdtsc_ordered() returns the result of RDTSC as a 64-bit integer.
+ * It is ordered like a load to a global in-memory counter.  It should
+ * be impossible to observe non-monotonic rdtsc_unordered() behavior
+ * across multiple CPUs as long as the TSC is synced.
+ */
+static __always_inline unsigned long long rdtsc_ordered(void)
+{
+	barrier_before_rdtsc();
+	return rdtsc_unordered();
+}
+
 static inline unsigned long long native_read_pmc(int counter)
 {
 	DECLARE_ARGS(val, low, high);
diff --git a/arch/x86/kernel/trace_clock.c b/arch/x86/kernel/trace_clock.c
index 60ddc0501beb..80bb24d9b880 100644
--- a/arch/x86/kernel/trace_clock.c
+++ b/arch/x86/kernel/trace_clock.c
@@ -12,10 +12,5 @@
  */
 u64 notrace trace_clock_x86_tsc(void)
 {
-	u64 ret;
-
-	barrier_before_rdtsc();
-	ret = rdtsc_unordered();
-
-	return ret;
+	return rdtsc_ordered();
 }
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index dbd2e5b4346a..ba69c06d8150 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1419,20 +1419,8 @@ EXPORT_SYMBOL_GPL(kvm_write_tsc);
 
 static cycle_t read_tsc(void)
 {
-	cycle_t ret;
-	u64 last;
-
-	/*
-	 * Empirically, a fence (of type that depends on the CPU)
-	 * before rdtsc is enough to ensure that rdtsc is ordered
-	 * with respect to loads.  The various CPU manuals are unclear
-	 * as to whether rdtsc can be reordered with later loads,
-	 * but no one has ever seen it happen.
-	 */
-	barrier_before_rdtsc();
-	ret = (cycle_t)rdtsc_unordered();
-
-	last = pvclock_gtod_data.clock.cycle_last;
+	cycle_t ret = (cycle_t)rdtsc_ordered();
+	u64 last = pvclock_gtod_data.clock.cycle_last;
 
 	if (likely(ret >= last))
 		return ret;
diff --git a/arch/x86/lib/delay.c b/arch/x86/lib/delay.c
index d49b98a192ae..82bd6aaafe72 100644
--- a/arch/x86/lib/delay.c
+++ b/arch/x86/lib/delay.c
@@ -54,11 +54,9 @@ static void delay_tsc(unsigned long __loops)
 
 	preempt_disable();
 	cpu = smp_processor_id();
-	barrier_before_rdtsc();
-	bclock = rdtsc_unordered();
+	bclock = rdtsc_ordered();
 	for (;;) {
-		barrier_before_rdtsc();
-		now = rdtsc_unordered();
+		now = rdtsc_ordered();
 		if ((now - bclock) >= loops)
 			break;
 
@@ -79,8 +77,7 @@ static void delay_tsc(unsigned long __loops)
 		if (unlikely(cpu != smp_processor_id())) {
 			loops -= (now - bclock);
 			cpu = smp_processor_id();
-			barrier_before_rdtsc();
-			bclock = rdtsc_unordered();
+			bclock = rdtsc_ordered();
 		}
 	}
 	preempt_enable();
-- 
2.4.2

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