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.

Signed-off-by: Andrea della Porta <andrea.porta@suse.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)
-- 
2.35.3

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@suse.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)

On 12:56 Tue 24 Oct     , Robin Murphy wrote:
> 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

You're right, I guess we can simpluy leverage system_supports_32bit_el0()
calling id_aa64pfr0_32bit_el0() and override the el0 nibble from command line,
instead of inventing a new kernel parameter. For the sake of simplicity,
maybe we can add a new alias in idreg-override, something like 'arm64.no32bit-el0'.

Thanks,
Andrea