On 30/08/2022 13.56, Nico Boehr wrote:
Upcoming changes will do performance measurements of instructions. Since
stck is designed to return unique values even on concurrent calls, it is
unsuited for performance measurements. stckf should be used in this
case.
Hence, add a nice wrapper for stckf to the time library.
Signed-off-by: Nico Boehr <nrb@xxxxxxxxxxxxx>
---
lib/s390x/asm/time.h | 9 +++++++++
1 file changed, 9 insertions(+)
diff --git a/lib/s390x/asm/time.h b/lib/s390x/asm/time.h
index 7652a151e87a..d7c2bcb4f306 100644
--- a/lib/s390x/asm/time.h
+++ b/lib/s390x/asm/time.h
@@ -14,6 +14,15 @@
#define STCK_SHIFT_US (63 - 51)
#define STCK_MAX ((1UL << 52) - 1)
+static inline uint64_t get_clock_fast(void)
+{
+ uint64_t clk;
+
+ asm volatile(" stckf %0 " : : "Q"(clk) : "memory");
+
+ return clk;
+}
Using clk as input parameter together with memory clobbing sounds like a bad
solution to me here. The Linux kernel properly uses it as output parameter
instead:
static inline unsigned long get_tod_clock_fast(void)
{
unsigned long clk;
asm volatile("stckf %0" : "=Q" (clk) : : "cc");
return clk;
}
(see arch/s390/include/asm/timex.h in the kernel sources)
As you can see, also the "cc" should be in the clobber list here.
Thomas
