kselftest/arm64: Verify the GCS hwcap

Add coverage of the GCS hwcap to the hwcap selftest, using a read of
GCSPR_EL0 to generate SIGILL without having to worry about enabling GCS.

Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
Tested-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20241001-arm64-gcs-v13-29-222b78d87eee@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Author: broonie

tools/testing/selftests/arm64/abi/hwcap.c

@@ -98,6 +98,17 @@ static void fpmr_sigill(void)
 	asm volatile("mrs x0, S3_3_C4_C4_2" : : : "x0");
 }
 
+static void gcs_sigill(void)
+{
+	unsigned long *gcspr;
+
+	asm volatile(
+		"mrs	%0, S3_3_C2_C5_1"
+	: "=r" (gcspr)
+	:
+	: "cc");
+}
+
 static void ilrcpc_sigill(void)
 {
 	/* LDAPUR W0, [SP, #8] */
@@ -534,6 +545,14 @@ static const struct hwcap_data {
 		.sigill_fn = fpmr_sigill,
 		.sigill_reliable = true,
 	},
+	{
+		.name = "GCS",
+		.at_hwcap = AT_HWCAP,
+		.hwcap_bit = HWCAP_GCS,
+		.cpuinfo = "gcs",
+		.sigill_fn = gcs_sigill,
+		.sigill_reliable = true,
+	},
 	{
 		.name = "JSCVT",
 		.at_hwcap = AT_HWCAP,