[PATCH] arm64: set MITIGATE_SPECTRE_BRANCH_HISTORY to n by default

[Will Deacon]

On Tue, Jul 16, 2024 at 05:51:36PM +0900, Chanho Min wrote:
> lmbench shows 25% performance regression after MITIGATE_SPECTRE_BRANCH_HISTORY
> is enabled. This performance drop may be more significant than mitigating
> the spectre-BHB. This patch changes this security option to disable by default
> and makes it selectable.
> 
> - lat_syscall result with MITIGATE_SPECTRE_BRANCH_HISTORY enabled (cortex-a78)
> write call      0.2777  0.2810  0.2824  0.280367
> read call       0.3435  0.3452  0.3443  0.344333
> 
> - lat_syscall result with MITIGATE_SPECTRE_BRANCH_HISTORY disabled (cortex-a78)
> write call      0.2101  0.2117  0.2116  0.2111
> read call       0.2732  0.2744  0.2763  0.274633

It's sad, but it's not surprising. Mitigations rarely make things faster.

> Signed-off-by: Chanho Min <chanho.min at lge.com>
> ---
>  arch/arm64/Kconfig | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 5d91259ee7b5..be76f425c060 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -1626,7 +1626,7 @@ config UNMAP_KERNEL_AT_EL0
>  
>  config MITIGATE_SPECTRE_BRANCH_HISTORY
>  	bool "Mitigate Spectre style attacks against branch history" if EXPERT
> -	default y
> +	default n
>  	help
>  	  Speculation attacks against some high-performance processors can
>  	  make use of branch history to influence future speculation.
> -- 
> 2.17.1

It's better to be safe than sorry, so I think it's right to default
this to 'y'. The mitigation can be disabled on the kernel command-line,
e.g. using the "nospectre_bhb" option.

Will