Subject: Re: [PATCH v4 02/14] arm64: Allow mismatched 32-bit EL0 support

From: Marc Zyngier <[email-masked]>
Date: Fri, 27 Nov 2020 10:25:06 +0000

On 2020-11-24 15:50, Will Deacon wrote:
> When confronted with a mixture of CPUs, some of which support 32-bit
> applications and others which don't, we quite sensibly treat the system
> as 64-bit only for userspace and prevent execve() of 32-bit binaries.
> 
> Unfortunately, some crazy folks have decided to build systems like this
> with the intention of running 32-bit applications, so relax our
> sanitisation logic to continue to advertise 32-bit support to userspace
> on these systems and track the real 32-bit capable cores in a cpumask
> instead. For now, the default behaviour remains but will be tied to
> a command-line option in a later patch.
> 
> Signed-off-by: Will Deacon <[email-masked]>
> ---
>  arch/arm64/include/asm/cpucaps.h    |   2 +-
>  arch/arm64/include/asm/cpufeature.h |   8 ++-
>  arch/arm64/kernel/cpufeature.c      | 106 ++++++++++++++++++++++++++--
>  3 files changed, 107 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/cpucaps.h 
> b/arch/arm64/include/asm/cpucaps.h
> index e7d98997c09c..e6f0eb4643a0 100644
> --- a/arch/arm64/include/asm/cpucaps.h
> +++ b/arch/arm64/include/asm/cpucaps.h
> @@ -20,7 +20,7 @@
>  #define ARM64_ALT_PAN_NOT_UAO			10
>  #define ARM64_HAS_VIRT_HOST_EXTN		11
>  #define ARM64_WORKAROUND_CAVIUM_27456		12
> -#define ARM64_HAS_32BIT_EL0			13
> +#define ARM64_HAS_32BIT_EL0_DO_NOT_USE		13
>  #define ARM64_HARDEN_EL2_VECTORS		14
>  #define ARM64_HAS_CNP				15
>  #define ARM64_HAS_NO_FPSIMD			16
> diff --git a/arch/arm64/include/asm/cpufeature.h
> b/arch/arm64/include/asm/cpufeature.h
> index 97244d4feca9..f447d313a9c5 100644
> --- a/arch/arm64/include/asm/cpufeature.h
> +++ b/arch/arm64/include/asm/cpufeature.h
> @@ -604,9 +604,15 @@ static inline bool 
> cpu_supports_mixed_endian_el0(void)
>  	return id_aa64mmfr0_mixed_endian_el0(read_cpuid(ID_AA64MMFR0_EL1));
>  }
> 
> +const struct cpumask *system_32bit_el0_cpumask(void);
> +DECLARE_STATIC_KEY_FALSE(arm64_mismatched_32bit_el0);
> +
>  static inline bool system_supports_32bit_el0(void)
>  {
> -	return cpus_have_const_cap(ARM64_HAS_32BIT_EL0);
> +	u64 pfr0 = read_sanitised_ftr_reg(SYS_ID_AA64PFR0_EL1);
> +
> +	return id_aa64pfr0_32bit_el0(pfr0) ||
> +	       static_branch_unlikely(&arm64_mismatched_32bit_el0);

nit: swapping the two sides of this expression has the potential
for slightly better code, resulting in better performance on
these asymmetric systems. Not a big real, but since this lands
on the fast path on vcpu exit, I'll take every bit of optimisation.

Thanks,

         M.
-- 
Jazz is not dead. It just smells funny...

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