arm64/sme: Implement sysctl to set the default vector length

As for SVE provide a sysctl which allows the default SME vector length to
be configured.

Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20220419112247.711548-11-broonie@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/fpsimd.c

@@ -489,6 +489,30 @@ static int __init sve_sysctl_init(void)
 static int __init sve_sysctl_init(void) { return 0; }
 #endif /* ! (CONFIG_ARM64_SVE && CONFIG_SYSCTL) */
 
+#if defined(CONFIG_ARM64_SME) && defined(CONFIG_SYSCTL)
+static struct ctl_table sme_default_vl_table[] = {
+	{
+		.procname	= "sme_default_vector_length",
+		.mode		= 0644,
+		.proc_handler	= vec_proc_do_default_vl,
+		.extra1		= &vl_info[ARM64_VEC_SME],
+	},
+	{ }
+};
+
+static int __init sme_sysctl_init(void)
+{
+	if (system_supports_sme())
+		if (!register_sysctl("abi", sme_default_vl_table))
+			return -EINVAL;
+
+	return 0;
+}
+
+#else /* ! (CONFIG_ARM64_SME && CONFIG_SYSCTL) */
+static int __init sme_sysctl_init(void) { return 0; }
+#endif /* ! (CONFIG_ARM64_SME && CONFIG_SYSCTL) */
+
 #define ZREG(sve_state, vq, n) ((char *)(sve_state) +		\
 	(SVE_SIG_ZREG_OFFSET(vq, n) - SVE_SIG_REGS_OFFSET))
 
@@ -1687,6 +1711,9 @@ static int __init fpsimd_init(void)
 	if (cpu_have_named_feature(SME) && !cpu_have_named_feature(SVE))
 		pr_notice("SME is implemented but not SVE\n");
 
-	return sve_sysctl_init();
+	sve_sysctl_init();
+	sme_sysctl_init();
+
+	return 0;
 }
 core_initcall(fpsimd_init);