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Date:   Fri, 27 Nov 2020 13:58:05 +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 00/14] An alternative series for asymmetric AArch32
 systems
Message-ID: <20201127135805.iubhh7albp3nsyxd@e107158-lin.cambridge.arm.com>
References: <20201124155039.13804-1-will@kernel.org>
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In-Reply-To: <20201124155039.13804-1-will@kernel.org>
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Hi Will

On 11/24/20 15:50, Will Deacon wrote:
> Hello folks,
> 
> Here's version four of the wonderful patches I previously posted here:
> 
>   v1: https://lore.kernel.org/r/20201027215118.27003-1-will@kernel.org
>   v2: https://lore.kernel.org/r/20201109213023.15092-1-will@kernel.org
>   v3: https://lore.kernel.org/r/20201113093720.21106-1-will@kernel.org
> 
> and which started life as a reimplementation of some patches from Qais:
> 
>   https://lore.kernel.org/r/20201021104611.2744565-1-qais.yousef@arm.com
> 
> The aim of this series is to allow 32-bit ARM applications to run on
> arm64 SoCs where not all of the CPUs support the 32-bit instruction set.
> Unfortunately, such SoCs are real and will continue to be productised
> over the next few years at least.
> 
> Changes in v4 include:
> 
>   * Take into account the cpuset 'allowed' mask on exec
>   * Print a warning if we forcefully override the affinity, like we do
>     in select_fallback_rq()
>   * Rename arch_cpu_allowed_mask() to arch_task_cpu_possible_mask()
>   * Added a comment to adjust_compat_task_affinity()

Thanks for that and sorry for the delayed response. I wanted to dig in sooner
but a combination of holiday and other matters kept me away longer than
I wanted.

I think this approach is definitely the most elegant option and something
Morten and myself tried to argue for on the first post. The new execve problem
now mandates this approach as it's hard to fix otherwise. The only other
suggestion I had is to extend execve syscall to accept a cgroup to attach the
task to. Similar to what we have in clone3 syscall. I think libc can be
modified then to ensure 32bit binaries execute into a cpuset with the right
affinity set. But that's less elegant approach.

I'm trying to run more tests on it, specially with regard to cpuset handling as
this is the trickiest bit. The approach and patches look good to me in general.
Most of my comments were minor remarks or nits.

My only worry is that this approach might be too elegant to deter these SoCs
from proliferating :-p

Cheers

--
Qais Yousef

> 
> Cheers,
> 
> Will
> 
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Marc Zyngier <maz@kernel.org>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Morten Rasmussen <morten.rasmussen@arm.com>
> Cc: Qais Yousef <qais.yousef@arm.com>
> Cc: Suren Baghdasaryan <surenb@google.com>
> Cc: Quentin Perret <qperret@google.com>
> Cc: Tejun Heo <tj@kernel.org>
> Cc: Li Zefan <lizefan@huawei.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Juri Lelli <juri.lelli@redhat.com>
> Cc: Vincent Guittot <vincent.guittot@linaro.org>
> Cc: kernel-team@android.com
> 
> --->8
> 
> Will Deacon (14):
>   arm64: cpuinfo: Split AArch32 registers out into a separate struct
>   arm64: Allow mismatched 32-bit EL0 support
>   KVM: arm64: Kill 32-bit vCPUs on systems with mismatched EL0 support
>   arm64: Kill 32-bit applications scheduled on 64-bit-only CPUs
>   arm64: Advertise CPUs capable of running 32-bit applications in sysfs
>   arm64: Hook up cmdline parameter to allow mismatched 32-bit EL0
>   sched: Introduce restrict_cpus_allowed_ptr() to limit task CPU
>     affinity
>   arm64: exec: Adjust affinity for compat tasks with mismatched 32-bit
>     EL0
>   cpuset: Don't use the cpu_possible_mask as a last resort for cgroup v1
>   sched: Introduce arch_task_cpu_possible_mask() to limit fallback rq
>     selection
>   sched: Reject CPU affinity changes based on
>     arch_task_cpu_possible_mask()
>   arm64: Prevent offlining first CPU with 32-bit EL0 on mismatched
>     system
>   arm64: Implement arch_task_cpu_possible_mask()
>   arm64: Remove logic to kill 32-bit tasks on 64-bit-only cores
> 
>  .../ABI/testing/sysfs-devices-system-cpu      |   9 +
>  .../admin-guide/kernel-parameters.txt         |   7 +
>  arch/arm64/include/asm/cpu.h                  |  44 ++--
>  arch/arm64/include/asm/cpucaps.h              |   2 +-
>  arch/arm64/include/asm/cpufeature.h           |   8 +-
>  arch/arm64/include/asm/mmu_context.h          |  13 ++
>  arch/arm64/kernel/cpufeature.c                | 219 ++++++++++++++----
>  arch/arm64/kernel/cpuinfo.c                   |  53 +++--
>  arch/arm64/kernel/process.c                   |  47 +++-
>  arch/arm64/kvm/arm.c                          |  11 +-
>  include/linux/sched.h                         |   1 +
>  kernel/cgroup/cpuset.c                        |   6 +-
>  kernel/sched/core.c                           |  90 +++++--
>  13 files changed, 401 insertions(+), 109 deletions(-)
> 
> -- 
> 2.29.2.454.gaff20da3a2-goog
>