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Re: [PATCH v3 3/7] arm64: HWCAP: encapsulate elf_hwcap
- From: Dave Martin <Dave dot Martin at arm dot com>
- To: Andrew Murray <andrew dot murray at arm dot com>
- Cc: Catalin Marinas <catalin dot marinas at arm dot com>, Will Deacon <will dot deacon at arm dot com>, Szabolcs Nagy <Szabolcs dot Nagy at arm dot com>, linux-arm-kernel at lists dot infradead dot org, Mark Rutland <mark dot rutland at arm dot com>, Phil Blundell <pb at pbcl dot net>, libc-alpha at sourceware dot org, linux-api at vger dot kernel dot org
- Date: Tue, 2 Apr 2019 15:58:21 +0100
- Subject: Re: [PATCH v3 3/7] arm64: HWCAP: encapsulate elf_hwcap
- References: <20190401104515.39775-1-andrew.murray@arm.com> <20190401104515.39775-4-andrew.murray@arm.com>
On Mon, Apr 01, 2019 at 11:45:11AM +0100, Andrew Murray wrote:
> The introduction of AT_HWCAP2 introduced accessors which ensure that
> hwcap features are set and tested appropriately.
>
> Let's now mandate access to elf_hwcap via these accessors by making
> elf_hwcap static within cpufeature.c.
Looks reasonable except for a couple of minor nits below.
I had wondered whether putting these accessors out of line would affect
any hot paths, but I can't see these used from anything that looks like
a hot path. So we're probably fine.
cpus_have_const_cap() is preferred for places where this matters,
anyway.
[...]
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 986ceeacd19f..84ca52fa75e5 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -35,8 +35,7 @@
> #include <asm/traps.h>
> #include <asm/virt.h>
>
> -unsigned long elf_hwcap __read_mostly;
> -EXPORT_SYMBOL_GPL(elf_hwcap);
> +static unsigned long elf_hwcap __read_mostly;
Now that this doesn't correspond directly to ELF_HWCAP any more and we
hide it, can we rename it to avoid confusion?
Maybe "kernel_hwcap"?
> #ifdef CONFIG_COMPAT
> #define COMPAT_ELF_HWCAP_DEFAULT \
> @@ -1947,6 +1946,35 @@ bool this_cpu_has_cap(unsigned int n)
> return false;
> }
>
> +void cpu_set_feature(unsigned int num)
> +{
> + WARN_ON(num >= MAX_CPU_FEATURES);
> + elf_hwcap |= BIT(num);
> +}
> +EXPORT_SYMBOL_GPL(cpu_set_feature);
> +
> +bool cpu_have_feature(unsigned int num)
> +{
> + WARN_ON(num >= MAX_CPU_FEATURES);
> + return elf_hwcap & BIT(num);
> +}
> +EXPORT_SYMBOL_GPL(cpu_have_feature);
> +
> +unsigned long cpu_get_elf_hwcap(void)
> +{
> + /*
> + * We currently only populate the first 32 bits of AT_HWCAP. Please
> + * note that for userspace compatibility we guarantee that bit 62
> + * will always be returned as 0.
> + */
Presumably also bit 63?
It is reasonable to say this here, but I think there should also be a
note in Documentation/arm64/elf_hwcaps.txt.
[...]
Cheers
---Dave