Re: [PATCH RFC v2 2/2] arm64: Support AT_HWCAP3

[Anshuman Khandual]

On 10/5/24 01:56, Mark Brown wrote:
> We have filled all 64 bits of AT_HWCAP2 so in order to support discovery of
> further features provide the framework to use the already defined AT_HWCAP3
> for further CPU features.
> 
> Signed-off-by: Mark Brown <broonie@xxxxxxxxxx>
> ---
>  Documentation/arch/arm64/elf_hwcaps.rst | 6 +++---
>  arch/arm64/include/asm/cpufeature.h     | 3 ++-
>  arch/arm64/include/asm/hwcap.h          | 6 +++++-
>  arch/arm64/include/uapi/asm/hwcap.h     | 4 ++++
>  arch/arm64/kernel/cpufeature.c          | 6 ++++++
>  5 files changed, 20 insertions(+), 5 deletions(-)
> 
> diff --git a/Documentation/arch/arm64/elf_hwcaps.rst b/Documentation/arch/arm64/elf_hwcaps.rst
> index 694f67fa07d196816b1292e896ebe6a1b599c125..dc1b11d6d1122bba928b054cd1874aad5073e05c 100644
> --- a/Documentation/arch/arm64/elf_hwcaps.rst
> +++ b/Documentation/arch/arm64/elf_hwcaps.rst
> @@ -16,9 +16,9 @@ architected discovery mechanism available to userspace code at EL0. The
>  kernel exposes the presence of these features to userspace through a set
>  of flags called hwcaps, exposed in the auxiliary vector.
>  
> -Userspace software can test for features by acquiring the AT_HWCAP or
> -AT_HWCAP2 entry of the auxiliary vector, and testing whether the relevant
> -flags are set, e.g.::
> +Userspace software can test for features by acquiring the AT_HWCAP,
> +AT_HWCAP2 or AT_HWCAP3 entry of the auxiliary vector, and testing
> +whether the relevant flags are set, e.g.::
>  
>  	bool floating_point_is_present(void)
>  	{
> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> index 3d261cc123c1e22ac7bc9cfcde463624c76b2084..38e7d1a44ea38ab0a06a6f22824ae023b128721b 100644
> --- a/arch/arm64/include/asm/cpufeature.h
> +++ b/arch/arm64/include/asm/cpufeature.h
> @@ -12,7 +12,7 @@
>  #include <asm/hwcap.h>
>  #include <asm/sysreg.h>
>  
> -#define MAX_CPU_FEATURES	128
> +#define MAX_CPU_FEATURES	192
>  #define cpu_feature(x)		KERNEL_HWCAP_ ## x
>  
>  #define ARM64_SW_FEATURE_OVERRIDE_NOKASLR	0
> @@ -438,6 +438,7 @@ void cpu_set_feature(unsigned int num);
>  bool cpu_have_feature(unsigned int num);
>  unsigned long cpu_get_elf_hwcap(void);
>  unsigned long cpu_get_elf_hwcap2(void);
> +unsigned long cpu_get_elf_hwcap3(void);
>  
>  #define cpu_set_named_feature(name) cpu_set_feature(cpu_feature(name))
>  #define cpu_have_named_feature(name) cpu_have_feature(cpu_feature(name))
> diff --git a/arch/arm64/include/asm/hwcap.h b/arch/arm64/include/asm/hwcap.h
> index a775adddecf25633e87d58fb9ac9e6293beac1b3..3b5c50df419ee94193a4d9e3a47516eb0739e89a 100644
> --- a/arch/arm64/include/asm/hwcap.h
> +++ b/arch/arm64/include/asm/hwcap.h
> @@ -159,17 +159,21 @@
>  #define KERNEL_HWCAP_SME_SF8DP2		__khwcap2_feature(SME_SF8DP2)
>  #define KERNEL_HWCAP_POE		__khwcap2_feature(POE)
>  
> +#define __khwcap3_feature(x)		(const_ilog2(HWCAP3_ ## x) + 128)
> +
>  /*
>   * This yields a mask that user programs can use to figure out what
>   * instruction set this cpu supports.
>   */
>  #define ELF_HWCAP		cpu_get_elf_hwcap()
>  #define ELF_HWCAP2		cpu_get_elf_hwcap2()
> +#define ELF_HWCAP3		cpu_get_elf_hwcap3()
>  
>  #ifdef CONFIG_COMPAT
>  #define COMPAT_ELF_HWCAP	(compat_elf_hwcap)
>  #define COMPAT_ELF_HWCAP2	(compat_elf_hwcap2)
> -extern unsigned int compat_elf_hwcap, compat_elf_hwcap2;
> +#define COMPAT_ELF_HWCAP3	(compat_elf_hwcap3)
> +extern unsigned int compat_elf_hwcap, compat_elf_hwcap2, compat_elf_hwcap3;
>  #endif
>  
>  enum {
> diff --git a/arch/arm64/include/uapi/asm/hwcap.h b/arch/arm64/include/uapi/asm/hwcap.h
> index 055381b2c61595361c2d57d38be936c2dfeaa195..3dd4a53a438a14bfb41882b2ab15bdd7ce617475 100644
> --- a/arch/arm64/include/uapi/asm/hwcap.h
> +++ b/arch/arm64/include/uapi/asm/hwcap.h
> @@ -124,4 +124,8 @@
>  #define HWCAP2_SME_SF8DP2	(1UL << 62)
>  #define HWCAP2_POE		(1UL << 63)
>  
> +/*
> + * HWCAP3 flags - for AT_HWCAP3
> + */
> +
>  #endif /* _UAPI__ASM_HWCAP_H */
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 718728a85430fad5151b73fa213a510efac3f834..7221636b790709b153b49126e00246cc3032a7bc 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -103,6 +103,7 @@ static DECLARE_BITMAP(elf_hwcap, MAX_CPU_FEATURES) __read_mostly;
>  				 COMPAT_HWCAP_LPAE)
>  unsigned int compat_elf_hwcap __read_mostly = COMPAT_ELF_HWCAP_DEFAULT;
>  unsigned int compat_elf_hwcap2 __read_mostly;
> +unsigned int compat_elf_hwcap3 __read_mostly;
>  #endif
>  
>  DECLARE_BITMAP(system_cpucaps, ARM64_NCAPS);
> @@ -3499,6 +3500,11 @@ unsigned long cpu_get_elf_hwcap2(void)
>  	return elf_hwcap[1];
>  }
>  
> +unsigned long cpu_get_elf_hwcap3(void)
> +{
> +	return elf_hwcap[2];
> +}
> +
>  static void __init setup_boot_cpu_capabilities(void)
>  {
>  	/*
> 

LGTM, but just curious do you have a upcoming feature to be added
in AT_HWCAP3 soon ?

Reviewed-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>