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

Marc Zyngier
2021-01-18T10:02:06Z

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@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@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