Re: [PATCH v6 12/21] arm64: cpufeature: Add an early command-line cpufeature override facility

[Will Deacon]

On Mon, Feb 01, 2021 at 11:56:28AM +0000, Marc Zyngier 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
> functionality.
> 
> Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx>
> Acked-by: David Brazdil <dbrazdil@xxxxxxxxxx>
> Reviewed-by: Catalin Marinas <catalin.marinas@xxxxxxx>
> ---
>  arch/arm64/kernel/Makefile         |   2 +-
>  arch/arm64/kernel/head.S           |   1 +
>  arch/arm64/kernel/idreg-override.c | 164 +++++++++++++++++++++++++++++
>  3 files changed, 166 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..3243e3ae9bd8 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_feature_override
>  	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..d8d0d3b25bc3
> --- /dev/null
> +++ b/arch/arm64/kernel/idreg-override.c
> @@ -0,0 +1,164 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Early cpufeature override framework
> + *
> + * Copyright (C) 2020 Google LLC
> + * Author: Marc Zyngier <maz@xxxxxxxxxx>
> + */
> +
> +#include <linux/ctype.h>
> +#include <linux/kernel.h>
> +#include <linux/libfdt.h>
> +
> +#include <asm/cacheflush.h>
> +#include <asm/setup.h>
> +
> +#define FTR_DESC_NAME_LEN	20
> +#define FTR_DESC_FIELD_LEN	10
> +
> +struct ftr_set_desc {
> +	char 				name[FTR_DESC_NAME_LEN];
> +	struct arm64_ftr_override	*override;
> +	struct {
> +		char			name[FTR_DESC_FIELD_LEN];
> +		u8			shift;
> +	} 				fields[];
> +};
> +
> +static const struct ftr_set_desc * const regs[] __initconst = {
> +};
> +
> +static char *cmdline_contains_option(const char *cmdline, const char *option)
> +{
> +	char *str = strstr(cmdline, option);
> +
> +	if ((str == cmdline || (str > cmdline && isspace(*(str - 1)))))
> +		return str;
> +
> +	return NULL;
> +}
> +
> +static int __init find_field(const char *cmdline,
> +			     const struct ftr_set_desc *reg, int f, u64 *v)
> +{
> +	char opt[FTR_DESC_NAME_LEN + FTR_DESC_FIELD_LEN + 2], *str;
> +	size_t len;
> +
> +	snprintf(opt, ARRAY_SIZE(opt), "%s.%s=", reg->name, reg->fields[f].name);
> +
> +	str = cmdline_contains_option(cmdline, opt);
> +	if (!str)
> +		return -1;
> +
> +	str += strlen(opt);
> +	len = strcspn(str, " ");

I'm absolutely terrified of string parsing in C, but just wondering why you
only ignore literal spaces here. I _think_ the full-fat cmdline parsing uses
isspace() to delimit the options.

Would it be possible to reuse any of the logic over in parse_args() to avoid
having to roll this ourselves?

Will