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SubjectRe: [PATCH v4 02/14] arm64: Allow mismatched 32-bit EL0 support
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 <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
> #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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