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

[Qais Yousef <qais.yousef@arm.com>]

On 11/24/20 15:50, Will Deacon wrote:
> When confronted with a mixture of CPUs, some of which support 32-bit

Confronted made me laugh, well chosen word! :D

For some reason made me think of this :p

	https://www.youtube.com/watch?v=NJbXPzSPzxc&t=1m33s

> 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 <will@kernel.org>
> ---
>  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

nit: would UNUSED be better here? Worth adding a comment as to why too?

>  #define ARM64_HARDEN_EL2_VECTORS		14
>  #define ARM64_HAS_CNP				15
>  #define ARM64_HAS_NO_FPSIMD			16

[...]

> +static bool has_32bit_el0(const struct arm64_cpu_capabilities *entry, int scope)
> +{
> +	if (!has_cpuid_feature(entry, scope))
> +		return allow_mismatched_32bit_el0;

If a user passes the command line by mistake on a 64bit only system, this will
return true. I'll be honest, I'm not entirely sure what the impact is. I get
lost in the features maze. It is nicely encapsulated, but hard to navigate for
the none initiated :-)

Thanks

--
Qais Yousef

> +
> +	if (scope == SCOPE_SYSTEM)
> +		pr_info("detected: 32-bit EL0 Support\n");
> +
> +	return true;
> +}
> +
>  static bool has_useable_gicv3_cpuif(const struct arm64_cpu_capabilities *entry, int scope)
>  {
>  	bool has_sre;
> @@ -1803,10 +1890,9 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
>  	},
>  #endif	/* CONFIG_ARM64_VHE */
>  	{
> -		.desc = "32-bit EL0 Support",
> -		.capability = ARM64_HAS_32BIT_EL0,
> +		.capability = ARM64_HAS_32BIT_EL0_DO_NOT_USE,
>  		.type = ARM64_CPUCAP_SYSTEM_FEATURE,
> -		.matches = has_cpuid_feature,
> +		.matches = has_32bit_el0,
>  		.sys_reg = SYS_ID_AA64PFR0_EL1,
>  		.sign = FTR_UNSIGNED,
>  		.field_pos = ID_AA64PFR0_EL0_SHIFT,
> @@ -2299,7 +2385,7 @@ static const struct arm64_cpu_capabilities compat_elf_hwcaps[] = {
>  	{},
>  };