perf: riscv: skip empty batches in counter start

Avoid unnecessary SBI calls when starting non-overflowed counters
in pmu_sbi_start_ovf_ctrs_sbi() by checking ctr_start_mask.

Signed-off-by: Yunhui Cui <cuiyunhui@bytedance.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20250804025110.11088-1-cuiyunhui@bytedance.com
Signed-off-by: Paul Walmsley <pjw@kernel.org>

Author: cuiyunhui

drivers/perf/riscv_pmu_sbi.c

@@ -877,8 +877,10 @@ static inline void pmu_sbi_start_ovf_ctrs_sbi(struct cpu_hw_events *cpu_hw_evt,
 	for (i = 0; i < BITS_TO_LONGS(RISCV_MAX_COUNTERS); i++) {
 		ctr_start_mask = cpu_hw_evt->used_hw_ctrs[i] & ~ctr_ovf_mask;
 		/* Start all the counters that did not overflow in a single shot */
-		sbi_ecall(SBI_EXT_PMU, SBI_EXT_PMU_COUNTER_START, i * BITS_PER_LONG, ctr_start_mask,
-			0, 0, 0, 0);
+		if (ctr_start_mask) {
+			sbi_ecall(SBI_EXT_PMU, SBI_EXT_PMU_COUNTER_START, i * BITS_PER_LONG,
+				  ctr_start_mask, 0, 0, 0, 0);
+		}
 	}
 
 	/* Reinitialize and start all the counter that overflowed */