Re: [PATCH 0/6] An alternative series for asymmetric AArch32 systems

From: Suren Baghdasaryan <surenb@google.com>
To: Will Deacon <will@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org, linux-arch@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>,
	Qais Yousef <qais.yousef@arm.com>,
	kernel-team <kernel-team@android.com>,
	Elliott Hughes <enh@google.com>
Subject: Re: [PATCH 0/6] An alternative series for asymmetric AArch32 systems
Date: Thu, 29 Oct 2020 11:42:04 -0700	[thread overview]
Message-ID: <CAJuCfpH2vZfH0mZMxukKvcs1jW7Udiu4P-5z4ZyDU1-JN3xMdg@mail.gmail.com> (raw)
In-Reply-To: <20201027215118.27003-1-will@kernel.org>

On Tue, Oct 27, 2020 at 2:51 PM Will Deacon <will@kernel.org> wrote:
>
> Hi all,
>
> I was playing around with the asymmetric AArch32 RFCv2 from Qais:
>
> https://lore.kernel.org/r/20201021104611.2744565-1-qais.yousef@arm.com
>
> and ended up writing my own implementation this afternoon. I think it's
> smaller, simpler and easier to work with. In particular:
>
>   * I got rid of the sysctl in favour of a plain cmdline parameter
>   * I don't have a new CPU capability
>   * I don't have a new thread flag
>   * I expose a cpumask to userspace via sysfs to identify the 32-bit CPUs
>
> Anyway, I don't think we should merge this stuff (other than the first patch)
> until we've figured out what's going on in Android, but I wanted to get
> this out as something which we might be able to build on.

Hi Will,
Thanks for posting this series. Just to provide some more background,
on Android, 64-bit apps are forked from zygote64 process and 32-bit
ones from zygote. So normally we could handle the issues with such
asymmetric architectures using cpuset cgroup and placing zygote
process (and consequently all its children) in a separate cgroup with
affinity mask that includes only 32-bit capable cores. We would have
to take care of the affinity mask for such tasks during task
migrations, but it's still doable from userspace. However there are
64-bit apps which fork 32-bit processes and that is the case which is
unclear how to handle without help from the kernel. Still discussing
possible solutions. CC'ing more people from Android to be in the loop.
Thanks,
Suren.

>
> 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: kernel-team@android.com
>
> --->8
>
> Qais Yousef (1):
>   KVM: arm64: Handle Asymmetric AArch32 systems
>
> Will Deacon (5):
>   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 applcations in sysfs
>   arm64: Hook up cmdline parameter to allow mismatched 32-bit EL0
>
>  .../ABI/testing/sysfs-devices-system-cpu      |  8 ++
>  .../admin-guide/kernel-parameters.txt         |  7 ++
>  arch/arm64/include/asm/cpufeature.h           |  3 +
>  arch/arm64/kernel/cpufeature.c                | 80 ++++++++++++++++++-
>  arch/arm64/kernel/process.c                   | 21 ++++-
>  arch/arm64/kernel/signal.c                    | 26 ++++++
>  arch/arm64/kvm/arm.c                          | 27 +++++++
>  7 files changed, 168 insertions(+), 4 deletions(-)
>
> --
> 2.29.0.rc2.309.g374f81d7ae-goog
>