[PATCH v2 1/3] arm64: Avoid enabling KPTI unnecessarily

[Will Deacon]

On Mon, Nov 27, 2023 at 01:00:51PM +0100, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb at kernel.org>
> 
> Commit 42c5a3b04bf6 refactored the KPTI init code in a way that results
> in the use of non-global kernel mappings even on systems that have no
> need for it, and even when KPTI has been disabled explicitly via the
> command line.
> 
> Ensure that this only happens when we have decided (based on the
> detected system-wide CPU features) that KPTI should be enabled.
> 
> Fixes: 42c5a3b04bf6 ("arm64: Split kpti_install_ng_mappings()")
> Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
> ---
>  arch/arm64/kernel/cpufeature.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 646591c67e7a..91d2d6714969 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -1839,6 +1839,10 @@ static int __init __kpti_install_ng_mappings(void *__unused)
>  
>  static void __init kpti_install_ng_mappings(void)
>  {
> +	/* Check whether KPTI is going to be used */
> +	if (!cpus_have_cap(ARM64_UNMAP_KERNEL_AT_EL0))
> +		return;

Why can't you use arm64_kernel_unmapped_at_el0() here?

Will