linux-kernel - Re: [PATCH v2 2/2] arm64: cpufeature: Pretend that Apple A10 family does not support 32-bit EL0

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Message-ID: <ZuhbuYsRk5yRoSx8@arm.com>
Date: Mon, 16 Sep 2024 17:24:25 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: Nick Chan <towinchenmi@...il.com>
Cc: Will Deacon <will@...nel.org>, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org, asahi@...ts.linux.dev,
	Marc Zyngier <maz@...nel.org>
Subject: Re: [PATCH v2 2/2] arm64: cpufeature: Pretend that Apple A10 family
 does not support 32-bit EL0

On Tue, Sep 17, 2024 at 12:00:10AM +0800, Nick Chan wrote:
> Catalin Marinas 於 2024/9/16 晚上11:30 寫道:
> > On Mon, Sep 16, 2024 at 09:41:12PM +0800, Nick Chan wrote:
> >> On 9/9/2024 17:10, Nick Chan wrote:
> >>> Trying to support 32-bit EL0 on a CPU that can only execute it in certain
> >>> states is a bad idea. The A10 family only supports 16KB page size anyway
> >>> so many AArch32 executables won't run anyways. Pretend that it does not
> >>> support 32-bit EL0 at all.
> > 
> > CONFIG_COMPAT depends on ARM64_4K_PAGES || EXPERT. Do we really need
> > these patches in case one enables EXPERT and tries to run 32-bit
> > binaries that never ran on 16K pages before?
> 
> The worst thing that can happen is the 32-bit process crashes with illegal
> instruction, the kernel will still be fine.

Yes, but that's not my point. By default you do not get CONFIG_COMPAT
enabled with CONFIG_ARM64_16K_PAGES. So these patches are not really
necessary (unless one enabled EXPERT and turns on CONFIG_COMPAT but
these are specialist cases that I don't care about).

-- 
Catalin