linux-kernel - Re: [PATCH v2 2/2] arm64: cpufeature: Pretend that Apple A10 family does not support 32-bit EL0

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Message-ID: <f908c9ca-8063-44f4-b534-ddfc067b98c2@gmail.com>
Date: Mon, 16 Sep 2024 21:41:12 +0800
From: Nick Chan <towinchenmi@...il.com>
To: Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Cc: asahi@...ts.linux.dev, Marc Zyngier <maz@...nel.org>
Subject: Re: [PATCH v2 2/2] arm64: cpufeature: Pretend that Apple A10 family
 does not support 32-bit EL0



On 9/9/2024 17:10, Nick Chan wrote:
> The Apple A10 family consists of physical performance and efficiency
> cores, and only one of them can be active at a given time depending on
> the current p-state. However, only the performance cores can execute
> 32-bit EL0. This results in logical cores that can only execute 32-bit
> EL0 in high p-states.
Further research shows that the MPIDR_EL1 values between the two core
types are different. And whether the two core type have any extra
differences is anyone's guess right now. So far, nothing seems to break
horribly without special workarounds for the MPIDR value (with cpufreq
enabled downstream) as:
1. There are no KVM, GIC, ACPI, PSCI or cpuidle
2. All CPUs switch P-mode and E-mode together

However, all of this is broken enough that this piece of code should go
into arch/arm64/kernel/cpu_errata.c, and also generate a
TAINT_CPU_OUT_OF_SPEC for these cursed CPUs.

> 
> Trying to support 32-bit EL0 on a CPU that can only execute it in certain
> states is a bad idea. The A10 family only supports 16KB page size anyway
> so many AArch32 executables won't run anyways. Pretend that it does not
> support 32-bit EL0 at all.
> 
> Signed-off-by: Nick Chan <towinchenmi@...il.com>
> ---
>  arch/arm64/kernel/cpufeature.c | 27 +++++++++++++++++++++++++++
>  1 file changed, 27 insertions(+)
> 
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 718728a85430..386698f42172 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -3529,6 +3529,31 @@ void __init setup_boot_cpu_features(void)
>  	setup_boot_cpu_capabilities();
>  }
>  
> +static void __init bad_aarch32_el0_fixup(void)
> +{
> +	static const struct midr_range bad_aarch32_el0[] = {
> +		MIDR_ALL_VERSIONS(MIDR_APPLE_A10_T2_HURRICANE_ZEPHYR),
> +		MIDR_ALL_VERSIONS(MIDR_APPLE_A10X_HURRICANE_ZEPHYR),
> +		{}
> +	};
> +
> +	/*
> +	 * The Apple A10 family can only execute 32-bit EL0 when in high
> +	 * p-states. Pretend it does not support 32-bit EL0.
> +	 */
> +	if (is_midr_in_range_list(read_cpuid_id(), bad_aarch32_el0)) {
> +		struct arm64_ftr_reg *regp;
> +
> +		regp = get_arm64_ftr_reg(SYS_ID_AA64PFR0_EL1);
> +		if (!regp)
> +			return;
> +		u64 val = (regp->sys_val & ~ID_AA64PFR0_EL1_EL0_MASK)
> +		  | ID_AA64PFR0_EL1_EL0_IMP;
> +
> +		update_cpu_ftr_reg(regp, val);
> +	}
> +}
> +
>  static void __init setup_system_capabilities(void)
>  {
>  	/*
> @@ -3562,6 +3587,8 @@ static void __init setup_system_capabilities(void)
>  
>  void __init setup_system_features(void)
>  {
> +	bad_aarch32_el0_fixup();
> +
>  	setup_system_capabilities();
>  
>  	kpti_install_ng_mappings();

Nick Chan