linux-kernel - Re: [PATCH v2 1/4] arm64: Introduce aarch32

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Message-ID: <e13d8d85-67d7-4a5e-893c-69dacb3a6d24@arm.com>
Date:   Tue, 24 Oct 2023 12:56:12 +0100
From:   Robin Murphy <robin.murphy@....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
Cc:     nik.borisov@...e.com, arnd@...db.de, mark.rutland@....com
Subject: Re: [PATCH v2 1/4] arm64: Introduce aarch32_enabled()

On 23/10/2023 3:42 pm, Andrea della Porta wrote:
> Aarch32 bit support on 64bit kernels depends on whether CONFIG_COMPAT
> is selected or not. As it is a compile time option it doesn't
> provide the flexibility to have distributions set their own policy for
> Aarch32 support and give the user the flexibility to override it.
> 
> As a first step introduce aarch32_enabled() which abstracts whether 32
> bit compat is turned on or off. Upcoming patches will implement
> the ability to set Aarch32 compat state at boot time.

Other than patch #3, which as previously mentioned should be unnecessary 
if the kernel correctly never starts an "unsupported" AArch32 process to 
begin with, what does this do that simply overriding ID_AA64PFR0_EL1.EL0 
via the existing idreg-override mechanism wouldn't?

Thanks,
Robin.

> Signed-off-by: Andrea della Porta <andrea.porta@...e.com>
> ---
>   arch/arm64/include/asm/cpufeature.h | 15 +++++++++++++++
>   arch/arm64/kernel/entry-common.c    |  2 ++
>   2 files changed, 17 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> index 396af9b9c857..1180d68daaff 100644
> --- a/arch/arm64/include/asm/cpufeature.h
> +++ b/arch/arm64/include/asm/cpufeature.h
> @@ -657,6 +657,21 @@ static inline bool supports_clearbhb(int scope)
>   						    ID_AA64ISAR2_EL1_CLRBHB_SHIFT);
>   }
>   
> +#ifdef CONFIG_COMPAT
> +extern bool __aarch32_enabled;
> +
> +static inline bool aarch32_enabled(void)
> +{
> +	return __aarch32_enabled;
> +}
> +#else /* !CONFIG_COMPAT */
> +
> +static inline bool aarch32_enabled(void)
> +{
> +	return false;
> +}
> +#endif
> +
>   const struct cpumask *system_32bit_el0_cpumask(void);
>   DECLARE_STATIC_KEY_FALSE(arm64_mismatched_32bit_el0);
>   
> diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c
> index 0fc94207e69a..69ff9b8c0bde 100644
> --- a/arch/arm64/kernel/entry-common.c
> +++ b/arch/arm64/kernel/entry-common.c
> @@ -877,6 +877,8 @@ asmlinkage void noinstr el0t_32_error_handler(struct pt_regs *regs)
>   {
>   	__el0_error_handler_common(regs);
>   }
> +
> +bool __aarch32_enabled __ro_after_init = true;
>   #else /* CONFIG_COMPAT */
>   UNHANDLED(el0t, 32, sync)
>   UNHANDLED(el0t, 32, irq)