[PATCH v4 12/21] arm64: cpufeature: Add an early command-line cpufeature override facility

[Marc Zyngier]

In order to be able to override CPU features at boot time,
let's add a command line parser that matches options of the
form "cpureg.feature=value", and store the corresponding
value into the override val/mask pair.

No features are currently defined, so no expected change in
functionality.

Signed-off-by: Marc Zyngier <maz at kernel.org>
---
 arch/arm64/kernel/Makefile         |   2 +-
 arch/arm64/kernel/head.S           |   1 +
 arch/arm64/kernel/idreg-override.c | 119 +++++++++++++++++++++++++++++
 3 files changed, 121 insertions(+), 1 deletion(-)
 create mode 100644 arch/arm64/kernel/idreg-override.c

diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
index 86364ab6f13f..2262f0392857 100644
--- a/arch/arm64/kernel/Makefile
+++ b/arch/arm64/kernel/Makefile
@@ -17,7 +17,7 @@ obj-y			:= debug-monitors.o entry.o irq.o fpsimd.o		\
 			   return_address.o cpuinfo.o cpu_errata.o		\
 			   cpufeature.o alternative.o cacheinfo.o		\
 			   smp.o smp_spin_table.o topology.o smccc-call.o	\
-			   syscall.o proton-pack.o
+			   syscall.o proton-pack.o idreg-override.o
 
 targets			+= efi-entry.o
 
diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
index d74e5f84042e..b3c4dd04f74b 100644
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S
@@ -435,6 +435,7 @@ SYM_FUNC_START_LOCAL(__primary_switched)
 
 	mov	x0, x21				// pass FDT address in x0
 	bl	early_fdt_map			// Try mapping the FDT early
+	bl	init_shadow_regs
 	bl	switch_to_vhe
 #if defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_KASAN_SW_TAGS)
 	bl	kasan_early_init
diff --git a/arch/arm64/kernel/idreg-override.c b/arch/arm64/kernel/idreg-override.c
new file mode 100644
index 000000000000..392f93b67103
--- /dev/null
+++ b/arch/arm64/kernel/idreg-override.c
@@ -0,0 +1,119 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Early cpufeature override framework
+ *
+ * Copyright (C) 2020 Google LLC
+ * Author: Marc Zyngier <maz at kernel.org>
+ */
+
+#include <linux/kernel.h>
+#include <linux/libfdt.h>
+
+#include <asm/cacheflush.h>
+#include <asm/setup.h>
+
+struct reg_desc {
+	const char * const	name;
+	u64 * const		val;
+	u64 * const		mask;
+	struct {
+		const char * const	name;
+		u8			 shift;
+	} 			fields[];
+};
+
+static const struct reg_desc * const regs[] __initdata = {
+};
+
+static int __init find_field(const char *cmdline, const struct reg_desc *reg,
+			     int f, u64 *v)
+{
+	char buf[256], *str;
+	size_t len;
+
+	snprintf(buf, ARRAY_SIZE(buf), "%s.%s=", reg->name, reg->fields[f].name);
+
+	str = strstr(cmdline, buf);
+	if (!(str == cmdline || (str > cmdline && *(str - 1) == ' ')))
+		return -1;
+
+	str += strlen(buf);
+	len = strcspn(str, " ");
+	len = min(len, ARRAY_SIZE(buf) - 1);
+	strncpy(buf, str, len);
+	buf[len] = 0;
+
+	return kstrtou64(buf, 0, v);
+}
+
+static void __init match_options(const char *cmdline)
+{
+	int i;
+
+	for (i = 0; i < ARRAY_SIZE(regs); i++) {
+		int f;
+
+		if (!regs[i]->val || !regs[i]->mask)
+			continue;
+
+		for (f = 0; regs[i]->fields[f].name; f++) {
+			u64 v;
+
+			if (find_field(cmdline, regs[i], f, &v))
+				continue;
+
+			*regs[i]->val  |= (v & 0xf) << regs[i]->fields[f].shift;
+			*regs[i]->mask |= 0xfUL << regs[i]->fields[f].shift;
+		}
+	}
+}
+
+static __init void parse_cmdline(void)
+{
+	if (!IS_ENABLED(CONFIG_CMDLINE_FORCE)) {
+		const u8 *prop;
+		void *fdt;
+		int node;
+
+		fdt = get_early_fdt_ptr();
+		if (!fdt)
+			goto out;
+
+		node = fdt_path_offset(fdt, "/chosen");
+		if (node < 0)
+			goto out;
+
+		prop = fdt_getprop(fdt, node, "bootargs", NULL);
+		if (!prop)
+			goto out;
+
+		match_options(prop);
+
+		if (!IS_ENABLED(CONFIG_CMDLINE_EXTEND))
+			return;
+	}
+
+out:
+	match_options(CONFIG_CMDLINE);
+}
+
+void __init init_shadow_regs(void)
+{
+	int i;
+
+	for (i = 0; i < ARRAY_SIZE(regs); i++) {
+		if (regs[i]->val)
+			*regs[i]->val  = 0;
+		if (regs[i]->mask)
+			*regs[i]->mask = 0;
+	}
+
+	parse_cmdline();
+
+	for (i = 0; i < ARRAY_SIZE(regs); i++) {
+		if (regs[i]->val)
+			__flush_dcache_area(regs[i]->val, sizeof(*regs[i]->val));
+		if (regs[i]->mask)
+			__flush_dcache_area(regs[i]->mask, sizeof(*regs[i]->mask));
+	}
+}
-- 
2.29.2