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Date:   Fri, 27 Nov 2020 13:17:59 +0000
From:   Qais Yousef <qais.yousef@arm.com>
To:     Will Deacon <will@kernel.org>
Cc:     linux-arm-kernel@lists.infradead.org, linux-arch@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Catalin Marinas <catalin.marinas@arm.com>,
        Marc Zyngier <maz@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Morten Rasmussen <morten.rasmussen@arm.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Quentin Perret <qperret@google.com>, Tejun Heo <tj@kernel.org>,
        Li Zefan <lizefan@huawei.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Ingo Molnar <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        kernel-team@android.com
Subject: Re: [PATCH v4 06/14] arm64: Hook up cmdline parameter to allow
 mismatched 32-bit EL0
Message-ID: <20201127131759.5o5qiv2uwtb6qadl@e107158-lin.cambridge.arm.com>
References: <20201124155039.13804-1-will@kernel.org>
 <20201124155039.13804-7-will@kernel.org>
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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