[RFC][PATCH 2/9] time/sched_clock: Provide sched_clock_noinstr()

[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_noinstr() function.

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

As such, pull out the preempt_{dis,en}able_notrace() requirements from
the sched_clock_read() implementations by explicitly providing it in
the sched_clock() function.

This further requires said sched_clock_read() functions to be noinstr
themselves, for ARCH_WANTS_NO_INSTR users. See the next few patches.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 kernel/time/sched_clock.c |   22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

--- a/kernel/time/sched_clock.c
+++ b/kernel/time/sched_clock.c
@@ -64,7 +64,7 @@ static struct clock_data cd ____cachelin
 	.actual_read_sched_clock = jiffy_sched_clock_read,
 };
 
-static inline u64 notrace cyc_to_ns(u64 cyc, u32 mult, u32 shift)
+static __always_inline u64 cyc_to_ns(u64 cyc, u32 mult, u32 shift)
 {
 	return (cyc * mult) >> shift;
 }
@@ -80,23 +80,33 @@ notrace int sched_clock_read_retry(unsig
 	return raw_read_seqcount_latch_retry(&cd.seq, seq);
 }
 
-unsigned long long notrace sched_clock(void)
+unsigned long long noinstr sched_clock_noinstr(void)
 {
-	u64 cyc, res;
-	unsigned int seq;
 	struct clock_read_data *rd;
+	unsigned int seq;
+	u64 cyc, res;
 
 	do {
-		rd = sched_clock_read_begin(&seq);
+		seq = raw_read_seqcount_latch(&cd.seq);
+		rd = cd.read_data + (seq & 1);
 
 		cyc = (rd->read_sched_clock() - rd->epoch_cyc) &
 		      rd->sched_clock_mask;
 		res = rd->epoch_ns + cyc_to_ns(cyc, rd->mult, rd->shift);
-	} while (sched_clock_read_retry(seq));
+	} while (raw_read_seqcount_latch_retry(&cd.seq, seq));
 
 	return res;
 }
 
+unsigned long long notrace sched_clock(void)
+{
+	unsigned long long ns;
+	preempt_disable_notrace();
+	ns = sched_clock_noinstr();
+	preempt_enable_notrace();
+	return ns;
+}
+
 /*
  * Updating the data required to read the clock.
  *