[PATCH v4 13/21] arm64: entry: sdei: Make 'tsk' available

[Vladimir Murzin]

On 9/8/26 16:17, Mark Rutland wrote:
> For regular entry/exit sequences, we use 'tsk' (x28) to hold the current
> task pointer, allowing this to be used by various assembly macros. The
> SDEI entry/exit sequence uses x28 to hold the value of the interrupted
> sp_el0, and doesn't retain the current task pointer in a register.
> 
> These differences makes it awkward to share assembly macros across
> regular entry/exit and SDEI entry/exit, and risk surprises.
> 
> Align the SDEI entry/exit sequence with regular entry/exit, keeping the
> current task pointer in 'tsk', and preserving the interrupted sp_el0 in
> another (callee-saved) register. I've used x20 as x19 was already in
> used for the relevant sdei_registered_event, and these are the
> lowest-numbered registers above x18 (which we must preserve for shadow
> call stack).
> 
> Signed-off-by: Mark Rutland <mark.rutland at arm.com>
> Tested-by: Muhammad Usama Anjum <usama.anjum at arm.com>
> Cc: Ada Couprie Diaz <ada.coupriediaz at arm.com>
> Cc: Ard Biesheuvel <ardb at kernel.org>
> Cc: Catalin Marinas <catalin.marinas at arm.com>
> Cc: James Morse <james.morse at arm.com>
> Cc: Jinjie Ruan <ruanjinjie at huawei.com>
> Cc: Marc Zyngier <maz at kernel.org>
> Cc: Peter Zijlstra <peterz at infradead.org>
> Cc: Vladimir Murzin <vladimir.murzin at arm.com>
> Cc: Will Deacon <will at kernel.org>
> Cc: Yang Shi <yang at os.amperecomputing.com>
> ---
>  arch/arm64/kernel/entry.S | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index a6ec5c5a2d29c..0902c1bd9dd3d 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -1023,11 +1023,12 @@ SYM_CODE_START(__sdei_asm_handler)
>  
>  	/*
>  	 * We may have interrupted userspace, or a guest, or exit-from or
> -	 * return-to either of these. We can't trust sp_el0, restore it.
> +	 * return-to either of these. Preserve the interrupted sp_el0, and
> +	 * initialize sp_el0 to the current task.
>  	 */
> -	mrs	x28, sp_el0
> -	ldr_this_cpu	dst=x0, sym=__entry_task, tmp=x1
> -	msr	sp_el0, x0
> +	mrs	x20, sp_el0
> +	ldr_this_cpu    dst=tsk, sym=__entry_task, tmp=x1
> +	msr	sp_el0, tsk
>  
>  	/* If we interrupted the kernel point to the previous stack/frame. */
>  	and     x0, x3, #0xc
> @@ -1043,7 +1044,7 @@ SYM_CODE_START(__sdei_asm_handler)
>  	mov	x1, x19
>  	bl	__sdei_handler
>  
> -	msr	sp_el0, x28
> +	msr	sp_el0, x20
>  	/* restore regs >x17 that firmware won't restore */
>  	mov	x4, x19         // keep x4 for __sdei_asm_exit_trampoline
>  	ldp	x18, x19, [x4, #SDEI_EVENT_INTREGS + 16 * 9]
> -- 2.30.2
> 

I'm sure you are already pretty much aware that Will's series touches
this code as well... :)  

FWIW,

Reviewed-by: Vladimir Murzin <vladimir.murzin at arm.com>