Re: [PATCH] arm64: scs: Remove redundant save/restore of SCS SP on entry to/from EL0

[Mark Rutland <mark.rutland@arm.com>]

On Fri, Mar 13, 2026 at 12:32:19PM +0000, Will Deacon wrote:
> When returning to userspace, the SCS is empty and so the SCS SP just
> points to the base address of the SCS page.
> 
> Rather than saving and restoring this address in the current task, we
> can simply restore the SCS SP to point at the base of the stack on entry
> to EL1 from EL0.
> 
> Cc: Ard Biesheuvel <ardb@kernel.org>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Sami Tolvanen <samitolvanen@google.com>
> Signed-off-by: Will Deacon <will@kernel.org>

I think this works either way, so:

Acked-by: Mark Rutland <mark.rutland@arm.com>

Mark.

> ---
>  arch/arm64/include/asm/scs.h | 8 ++++++++
>  arch/arm64/kernel/entry.S    | 4 +---
>  2 files changed, 9 insertions(+), 3 deletions(-)
> 
> I just spotted this by inspection when I was hacking on our exception
> entry code.
> 
> diff --git a/arch/arm64/include/asm/scs.h b/arch/arm64/include/asm/scs.h
> index 0fbc2e7867d3..a15a2968e7b6 100644
> --- a/arch/arm64/include/asm/scs.h
> +++ b/arch/arm64/include/asm/scs.h
> @@ -10,6 +10,11 @@
>  #ifdef CONFIG_SHADOW_CALL_STACK
>  	scs_sp	.req	x18
>  
> +	.macro scs_load_current_base
> +	get_current_task scs_sp
> +	ldr	scs_sp, [scs_sp, #TSK_TI_SCS_BASE]
> +	.endm
> +
>  	.macro scs_load_current
>  	get_current_task scs_sp
>  	ldr	scs_sp, [scs_sp, #TSK_TI_SCS_SP]
> @@ -19,6 +24,9 @@
>  	str	scs_sp, [\tsk, #TSK_TI_SCS_SP]
>  	.endm
>  #else
> +	.macro scs_load_current_base
> +	.endm
> +
>  	.macro scs_load_current
>  	.endm
>  
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index f8018b5c1f9a..ab476ba060d1 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -273,7 +273,7 @@ alternative_if ARM64_HAS_ADDRESS_AUTH
>  alternative_else_nop_endif
>  1:
>  
> -	scs_load_current
> +	scs_load_current_base
>  	.else
>  	add	x21, sp, #PT_REGS_SIZE
>  	get_current_task tsk
> @@ -378,8 +378,6 @@ alternative_if ARM64_WORKAROUND_845719
>  alternative_else_nop_endif
>  #endif
>  3:
> -	scs_save tsk
> -
>  	/* Ignore asynchronous tag check faults in the uaccess routines */
>  	ldr	x0, [tsk, THREAD_SCTLR_USER]
>  	clear_mte_async_tcf x0
> -- 
> 2.53.0.851.ga537e3e6e9-goog
>