From: Qais Yousef <qais.yousef@arm.com>
To: Catalin Marinas <catalin.marinas@arm.com>
Cc: linux-arch@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Marc Zyngier <maz@kernel.org>, Will Deacon <will@kernel.org>,
Morten Rasmussen <morten.rasmussen@arm.com>,
linux-arm-kernel@lists.infradead.org
Subject: Re: [RFC PATCH 3/3] arm64: Handle AArch32 tasks running on non AArch32 cpu
Date: Tue, 13 Oct 2020 15:23:22 +0100 [thread overview]
Message-ID: <20201013142322.kldwvdnr2zjbia6e@e107158-lin> (raw)
In-Reply-To: <20201009124025.GH23638@gaia>
On 10/09/20 13:40, Catalin Marinas wrote:
> On Fri, Oct 09, 2020 at 12:31:56PM +0100, Qais Yousef wrote:
> > On 10/09/20 10:33, Catalin Marinas wrote:
> > > On Fri, Oct 09, 2020 at 09:31:47AM +0100, Will Deacon wrote:
> > > > Honestly, I don't understand why we're trying to hide this asymmetry from
> > > > userspace by playing games with affinity masks in the kernel. Userspace
> > > > is likely to want to move things about _anyway_ because even amongst the
> > > > 32-bit capable cores, you may well have different clock frequencies to
> > > > contend with.
> > > >
> > > > So I'd be *much* happier to let the schesduler do its thing, and if one
> > > > of these 32-bit tasks ends up on a core that can't deal with it, then
> > > > tough, it gets killed. Give userspace the information it needs to avoid
> > > > that happening in the first place, rather than implicitly limit the mask.
> > > >
> > > > That way, the kernel support really boils down to two parts:
> > > >
> > > > 1. Remove the sanity checks we have to prevent 32-bit applications running
> > > > on asymmetric systems
> > > >
> > > > 2. Tell userspace about the problem
> > >
> > > This works for me as well as long as it is default off with a knob to
> > > turn it on. I'd prefer a sysctl (which can be driven from the command
> > > line in recent kernels IIRC) so that one can play with it a run-time.
> > > This way it's also a userspace choice and not an admin or whoever
> > > controls the cmdline (well, that's rather theoretical since the target
> > > is Android).
> >
> > I like the cmdline option more. It implies a custom bootloader and user space
> > are required to enable this. Which in return implies they can write their own
> > custom driver to manage exporting this info to user-space. Reliefing us from
> > maintaining any ABI in mainline kernel.
>
> Regardless of whether it's cmdline or sysctl, I'm strongly opposed to
> custom drivers for exposing this information to user. It leads to
> custom incompatible ABIs scattered around.
Yeah I see what you mean.
So for now I'll remove the Kconfig dependency and replace it with a sysctl
instead such that system_supports_asym_32bit_el0() only returns true if the
system is asym AND the sysctl is true.
Thanks
--
Qais Yousef
> Note that user can already check the MIDR_EL1 value if it knows which
> CPU type and revision has 32-bit support.
>
> --
> Catalin
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next prev parent reply other threads:[~2020-10-13 14:24 UTC|newest]
Thread overview: 33+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-08 18:16 [RFC PATCH 0/3] Add support for Asymmetric AArch32 systems Qais Yousef
2020-10-08 18:16 ` [RFC PATCH 1/3] arm64: kvm: Handle " Qais Yousef
2020-10-09 8:12 ` Marc Zyngier
2020-10-09 9:58 ` Qais Yousef
2020-10-09 12:34 ` Marc Zyngier
2020-10-09 12:48 ` Qais Yousef
2020-10-12 15:32 ` James Morse
2020-10-13 10:32 ` Marc Zyngier
2020-10-13 11:51 ` James Morse
2020-10-13 11:59 ` Qais Yousef
2020-10-13 12:09 ` Marc Zyngier
2020-10-13 12:16 ` Qais Yousef
2020-10-08 18:16 ` [RFC PATCH 2/3] arm64: Add support for asymmetric AArch32 EL0 configurations Qais Yousef
2020-10-08 18:22 ` Randy Dunlap
2020-10-12 10:22 ` Qais Yousef
2020-10-09 6:13 ` Greg Kroah-Hartman
2020-10-09 8:40 ` Will Deacon
2020-10-09 8:50 ` Catalin Marinas
2020-10-09 9:39 ` Catalin Marinas
2020-10-12 12:46 ` Qais Yousef
2020-10-08 18:16 ` [RFC PATCH 3/3] arm64: Handle AArch32 tasks running on non AArch32 cpu Qais Yousef
2020-10-09 7:29 ` Peter Zijlstra
2020-10-09 8:13 ` Morten Rasmussen
2020-10-09 8:31 ` Will Deacon
2020-10-09 8:50 ` Morten Rasmussen
2020-10-09 9:33 ` Catalin Marinas
2020-10-09 9:42 ` Greg Kroah-Hartman
2020-10-09 11:31 ` Qais Yousef
2020-10-09 12:40 ` Catalin Marinas
2020-10-13 14:23 ` Qais Yousef [this message]
2020-10-09 9:25 ` Peter Zijlstra
2020-10-09 9:39 ` Qais Yousef
2020-10-09 9:51 ` Catalin Marinas
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