linux-kernel - Re: [PATCH 2/4] arm64/process: Make loading of 32bit processes depend on aarch32

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Message-ID: <ZTE86PK5YJGV1xVq@FVFF77S0Q05N.cambridge.arm.com>
Date:   Thu, 19 Oct 2023 15:27:52 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Andrea della Porta <andrea.porta@...e.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        nik.borisov@...e.com
Subject: Re: [PATCH 2/4] arm64/process: Make loading of 32bit processes
 depend on aarch32_enabled()

On Thu, Oct 19, 2023 at 02:38:32PM +0200, Andrea della Porta wrote:
> On 13:52 Wed 18 Oct     , Mark Rutland wrote:
> > On Wed, Oct 18, 2023 at 01:13:20PM +0200, Andrea della Porta wrote:
> > > Major aspect of Aarch32 emulation is the ability to load 32bit
> > > processes.
> > > That's currently decided (among others) by compat_elf_check_arch().
> > > 
> > > Make the macro use aarch32_enabled() to decide if Aarch32 compat is
> > > enabled before loading a 32bit process.
> > > 
> > > Signed-off-by: Andrea della Porta <andrea.porta@...e.com>
> > 
> > Why can't you make system_supports_32bit_el0() take the option into account
> > instead?
> >
> 
> I may be wrong here, but it seems to me that system_supports_32bit_el0()
> answers teh question "can this system supports compat execution?" rather than
> "do I want this system to run any compat execution?". That's the point of
> aarch32_enabled(), to state whether we want teh system to run A32 code or not,
> regardless of the system supporting it (of course, if the system does not
> support A32 in EL0, this is a no-no, but that's another story).

That's what the implementation does today, but we're really using it as a "do
we intend for 32-bit EL0 to work?" predicate, and generally the
system_supports_${FEATURE}() helpers are affected by the combination of actual
HW support, kernel config options, *and* kernel command line options. For
example, system_supports_sve() is affected by both CONFIG_ARM64_SVE and the
"arm64.nosve" command line option.

Thanks,
Mark.