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

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Date:   Fri, 05 Feb 2021 16:42:11 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Will Deacon <will@...nel.org>
Cc:     linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        David Brazdil <dbrazdil@...gle.com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Jing Zhang <jingzhangos@...gle.com>,
        Ajay Patil <pajay@....qualcomm.com>,
        Prasad Sodagudi <psodagud@...eaurora.org>,
        Srinivas Ramana <sramana@...eaurora.org>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        kernel-team@...roid.com
Subject: Re: [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@...nel.org>
>> Acked-by: David Brazdil <dbrazdil@...gle.com>
>> Reviewed-by: Catalin Marinas <catalin.marinas@....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@...nel.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...