[RFC PATCH v3 11/12] lib: vdso: split clock verification out of __arch_get_hw_counter()

Christophe Leroy christophe.leroy at c-s.fr
Mon Jan 13 09:08:49 PST 2020

__arch_get_hw_counter() returns the current value of the counter if
the counter is valid or a negative number if the counter is not valid.

This is suboptimal because the validity is checked twice: once before
reading the counter and once after reading the counter.

Optionaly split the verification out of __arch_get_hw_counter()
by providing an optional __arch_is_hw_counter_valid() function.

Signed-off-by: Christophe Leroy <christophe.leroy at c-s.fr>
 lib/vdso/gettimeofday.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/lib/vdso/gettimeofday.c b/lib/vdso/gettimeofday.c
index ea1a55507af5..001f6329e846 100644
--- a/lib/vdso/gettimeofday.c
+++ b/lib/vdso/gettimeofday.c
@@ -67,11 +67,18 @@ static __always_inline int do_hres(const struct vdso_data *vd, clockid_t clk,
 	do {
 		seq = vdso_read_begin(vd);
+#ifdef __arch_is_hw_counter_valid
+		if (unlikely(!__arch_is_hw_counter_valid(vd->clock_mode)))
+			return -1;
 		cycles = __arch_get_hw_counter(vd->clock_mode);
 		ns = vdso_ts->nsec;
 		last = vd->cycle_last;
+#ifndef __arch_is_hw_counter_valid
 		if (unlikely((s64)cycles < 0))
 			return -1;
 		ns += vdso_calc_delta(cycles, last, vd->mask, vd->mult);
 		nsec = ns >> vd->shift;

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