linux-kernel - Re: [PATCH 03/11] arm64: Take into account ID_AA64PFR0

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Date:   Fri, 5 Jan 2018 10:24:25 +0000
From:   Suzuki K Poulose <Suzuki.Poulose@....com>
To:     Will Deacon <will.deacon@....com>,
        linux-arm-kernel@...ts.infradead.org
Cc:     catalin.marinas@....com, ard.biesheuvel@...aro.org,
        marc.zyngier@....com, lorenzo.pieralisi@....com,
        christoffer.dall@...aro.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 03/11] arm64: Take into account ID_AA64PFR0_EL1.CSV3

On 04/01/18 15:08, Will Deacon wrote:
> For non-KASLR kernels where the KPTI behaviour has not been overridden
> on the command line we can use ID_AA64PFR0_EL1.CSV3 to determine whether
> or not we should unmap the kernel whilst running at EL0.
> 
> Signed-off-by: Will Deacon <will.deacon@....com>
> ---
>   arch/arm64/include/asm/sysreg.h | 1 +
>   arch/arm64/kernel/cpufeature.c  | 7 ++++++-
>   2 files changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/sysreg.h b/arch/arm64/include/asm/sysreg.h
> index 08cc88574659..ae519bbd3f9e 100644
> --- a/arch/arm64/include/asm/sysreg.h
> +++ b/arch/arm64/include/asm/sysreg.h
> @@ -437,6 +437,7 @@
>   #define ID_AA64ISAR1_DPB_SHIFT		0
>   
>   /* id_aa64pfr0 */
> +#define ID_AA64PFR0_CSV3_SHIFT		60
>   #define ID_AA64PFR0_SVE_SHIFT		32
>   #define ID_AA64PFR0_GIC_SHIFT		24
>   #define ID_AA64PFR0_ASIMD_SHIFT		20
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 9f0545dfe497..e11c11bb5b02 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -145,6 +145,7 @@ static const struct arm64_ftr_bits ftr_id_aa64isar1[] = {
>   };
>   
>   static const struct arm64_ftr_bits ftr_id_aa64pfr0[] = {
> +	ARM64_FTR_BITS(FTR_HIDDEN, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64PFR0_CSV3_SHIFT, 4, 0),
>   	ARM64_FTR_BITS(FTR_VISIBLE, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR0_SVE_SHIFT, 4, 0),
>   	ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR0_GIC_SHIFT, 4, 0),
>   	S_ARM64_FTR_BITS(FTR_VISIBLE, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR0_ASIMD_SHIFT, 4, ID_AA64PFR0_ASIMD_NI),
> @@ -851,6 +852,8 @@ static int __kpti_forced; /* 0: not forced, >0: forced on, <0: forced off */
>   static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
>   				int __unused)
>   {
> +	u64 pfr0 = read_sanitised_ftr_reg(SYS_ID_AA64PFR0_EL1);
> +
>   	/* Forced on command line? */
>   	if (__kpti_forced) {
>   		pr_info_once("kernel page table isolation forced %s by command line option\n",
> @@ -862,7 +865,9 @@ static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
>   	if (IS_ENABLED(CONFIG_RANDOMIZE_BASE))
>   		return true;
>   
> -	return false;
> +	/* Defer to CPU feature registers */
> +	return !cpuid_feature_extract_unsigned_field(pfr0,
> +						     ID_AA64PFR0_CSV3_SHIFT);
>   }
>   

The cpufeature bit changes look good to me. FWIW,

Reviewed-by: Suzuki K Poulose <suzuki.poulose@....com>