[PATCH v2 11/17] arm64: cpufeature: Add an early command-line cpufeature override facility

[Jing Zhang]

On Mon, Jan 4, 2021 at 8:20 AM Marc Zyngier <maz at kernel.org> wrote:
>
> 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
> functionnality.
Typo: functionnality -> 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));
> +       }
Could you shed some light on the usage of __flush_dcache_area here? Thanks.
> +}
> --
> 2.29.2
>
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