loongarch: Provide noinstr sched_clock_read()

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>
Tested-by: Michael Kelley <mikelley@microsoft.com>  # Hyper-V
Link: https://lore.kernel.org/r/20230519102715.502547082@infradead.org

arch/loongarch/include/asm/loongarch.h

@@ -1167,7 +1167,7 @@ static __always_inline void iocsr_write64(u64 val, u32 reg)
 
 #ifndef __ASSEMBLY__
 
-static inline u64 drdtime(void)
+static __always_inline u64 drdtime(void)
 {
 	int rID = 0;
 	u64 val = 0;

arch/loongarch/kernel/time.c

@@ -190,9 +190,9 @@ static u64 read_const_counter(struct clocksource *clk)
 	return drdtime();
 }
 
-static u64 native_sched_clock(void)
+static noinstr u64 sched_clock_read(void)
 {
-	return read_const_counter(NULL);
+	return drdtime();
 }
 
 static struct clocksource clocksource_const = {
@@ -211,7 +211,7 @@ int __init constant_clocksource_init(void)
 
 	res = clocksource_register_hz(&clocksource_const, freq);
 
-	sched_clock_register(native_sched_clock, 64, freq);
+	sched_clock_register(sched_clock_read, 64, freq);
 
 	pr_info("Constant clock source device register\n");