Re: [PATCH v2 4/4] perf tools: arm64: Add support for VG register

On Wed, May 18, 2022 at 10:44:57AM +0100, James Clark wrote:

[...]

quoted

quoted

+	if (getauxval(AT_HWCAP) & HWCAP_SVE)
+		attr.sample_regs_user |= SMPL_REG_MASK(PERF_REG_ARM64_VG);
+
+	/*
+	 * Check if the pmu supports perf extended regs, before
+	 * returning the register mask to sample.
+	 */
+	if (attr.sample_regs_user != PERF_REGS_MASK) {
+		event_attr_init(&attr);
+		fd = sys_perf_event_open(&attr, 0, -1, -1, 0);
+		if (fd != -1) {
+			close(fd);
+			return attr.sample_regs_user;
+		}
+	}

Just curious, since we can know SVE is supported from reading
auxiliary value, can we directly return the register mask as below?

  PERF_REGS_MASK | SMPL_REG_MASK(PERF_REG_ARM64_VG);

I was trying to cover the case where the system supports SVE, but
the kernel doesn't have my changes to add the VG register yet.

Technically I could just attempt to open the event without checking
for SVE first and see if it works or not. But I preferred to be
explicit so it's obvious why we're doing that.

Understand; LGTM.

Reviewed-by: Leo Yan <redacted>

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel