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SubjectRe: [PATCH v6 12/21] arm64: cpufeature: Add an early command-line cpufeature override facility
On 2021-02-05 16:35, Will Deacon wrote:
> 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@kernel.org>
>> Acked-by: David Brazdil <dbrazdil@google.com>
>> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
>> ---
>> 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@kernel.org>
>> + */
>> +
>> +#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.

That's clearly an oversight, as I use a more complete set of characters
for the rest of the option splicing. I also think that with the way
things are now parsed (options being extracted early and trimmed),
we can drop this altogether.

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

Maybe. I need to have a look.

Thanks,

M.
--
Jazz is not dead. It just smells funny...

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