Subject: Re: [PATCH v11 10/40] arm64/sme: Basic enumeration support

From: Catalin Marinas <[email-masked]>
Date: Mon, 21 Feb 2022 14:32:38 +0000

On Mon, Feb 07, 2022 at 03:20:39PM +0000, Mark Brown wrote:
> diff --git a/Documentation/arm64/elf_hwcaps.rst b/Documentation/arm64/elf_hwcaps.rst
> index b72ff17d600a..5626cf208000 100644
> --- a/Documentation/arm64/elf_hwcaps.rst
> +++ b/Documentation/arm64/elf_hwcaps.rst
> @@ -259,6 +259,39 @@ HWCAP2_RPRES
>  
>      Functionality implied by ID_AA64ISAR2_EL1.RPRES == 0b0001.
>  
> +HWCAP2_SME
> +
> +    Functionality implied by ID_AA64PFR1_EL1.SME == 0b0001, as described
> +    by Documentation/arm64/sme.rst.
> +
> +HWCAP2_SME_I16I64
> +
> +    Functionality implied by ID_AA64SMFR0_EL1.I16I64 == 0b1111.
> +
> +HWCAP2_SME_F64F64
> +
> +    Functionality implied by ID_AA64SMFR0_EL1.F64F64 == 0b1.
> +
> +HWCAP2_SME_I8I32
> +
> +    Functionality implied by ID_AA64SMFR0_EL1.I8I32 == 0b1111.
> +
> +HWCAP2_SME_F16F32
> +
> +    Functionality implied by ID_AA64SMFR0_EL1.F16F32 == 0b1.
> +
> +HWCAP2_SME_B16F32
> +
> +    Functionality implied by ID_AA64SMFR0_EL1.B16F32 == 0b1.
> +
> +HWCAP2_SME_F32F32
> +
> +    Functionality implied by ID_AA64SMFR0_EL1.F32F32 == 0b1.
> +
> +HWCAP2_SME_FA64
> +
> +    Functionality implied by ID_AA64SMFR0_EL1.FA64 == 0b1.

More of a question for the libc people: should we drop the fine-grained
HWCAP corresponding to the new ID_AA64SMFR0_EL1 register (only keep
HWCAP2_SME) and get the user space to use the MRS emulation? Would any
ifunc resolver be affected?

> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 64a748c2b351..2634e32bbfb9 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -251,6 +251,7 @@ static const struct arm64_ftr_bits ftr_id_aa64pfr0[] = {
>  };
>  
>  static const struct arm64_ftr_bits ftr_id_aa64pfr1[] = {
> +	ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR1_SME_SHIFT, 4, 0),

Shouldn't this field also be visible (if SME is enabled)?

> diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
> index 5280e098cfb5..576490be3c2b 100644
> --- a/arch/arm64/kernel/fpsimd.c
> +++ b/arch/arm64/kernel/fpsimd.c
> @@ -987,6 +987,32 @@ void fpsimd_release_task(struct task_struct *dead_task)
>  
>  #endif /* CONFIG_ARM64_SVE */
>  
> +#ifdef CONFIG_ARM64_SME
> +
> +void sme_kernel_enable(const struct arm64_cpu_capabilities *__always_unused p)
> +{
> +	/* Set priority for all PEs to architecturally defined minimum */
> +	write_sysreg_s(read_sysreg_s(SYS_SMPRI_EL1) & ~SMPRI_EL1_PRIORITY_MASK,
> +		       SYS_SMPRI_EL1);
> +
> +	/* Allow SME in kernel */
> +	write_sysreg(read_sysreg(CPACR_EL1) | CPACR_EL1_SMEN_EL1EN, CPACR_EL1);
> +	isb();
> +}
> +
> +/*
> + * This must be called after sme_kernel_enable(), we rely on the
> + * feature table being sorted to ensure this.
> + */
> +void fa64_kernel_enable(const struct arm64_cpu_capabilities *__always_unused p)
> +{
> +	/* Allow use of FA64 */
> +	write_sysreg_s(read_sysreg_s(SYS_SMCR_EL1) | SMCR_ELx_FA64_MASK,
> +		       SYS_SMCR_EL1);
> +}
> +
> +#endif /* CONFIG_ARM64_SVE */

I think instead of worrying about the order, we could check the
sanitised register value in sme_kernel_enable() and set the FA64 bit.
Also to me 'fa64_kernel_enable' somehow implies that the kernel cares
about FA64 for itself but AFAICT we never run the kernel in streaming
mode.

-- 
Catalin

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