Re: [PATCH v4 06/14] arm64: Hook up cmdline parameter to allow mismatched 32-bit EL0

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

On 11/24/20 15:50, Will Deacon wrote:
> Allow systems with mismatched 32-bit support at EL0 to run 32-bit
> applications based on a new kernel parameter.
> 
> Signed-off-by: Will Deacon <will@kernel.org>
> ---
>  Documentation/admin-guide/kernel-parameters.txt | 7 +++++++
>  arch/arm64/kernel/cpufeature.c                  | 7 +++++++
>  2 files changed, 14 insertions(+)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 526d65d8573a..f20188c44d83 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -289,6 +289,13 @@
>  			do not want to use tracing_snapshot_alloc() as it needs
>  			to be done where GFP_KERNEL allocations are allowed.
>  
> +	allow_mismatched_32bit_el0 [ARM64]
> +			Allow execve() of 32-bit applications and setting of the
> +			PER_LINUX32 personality on systems where only a strict
> +			subset of the CPUs support 32-bit EL0. When this
> +			parameter is present, the set of CPUs supporting 32-bit
> +			EL0 is indicated by /sys/devices/system/cpu/aarch32_el0.

Shouldn't we document that a randomly selected 32-bit CPU will be prevented
from being hotplugged out all the time to act as the last man standing for any
currently running 32-bit application.

That was a mouthful! I'm sure you can phrase it better :-)

If we make this the last patch as it was before adding affinity handling, we
can drop patch 4 more easily I think?

Thanks

--
Qais Yousef