linux-kernel - Re: [PATCH 3/6] arm64: scs: Use 'scs

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Message-ID: <20200518115547.GC1957@C02TD0UTHF1T.local>
Date:   Mon, 18 May 2020 12:55:47 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Will Deacon <will@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@...com>,
        Jann Horn <jannh@...gle.com>, Ard Biesheuvel <ardb@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, kernel-team@...roid.com
Subject: Re: [PATCH 3/6] arm64: scs: Use 'scs_sp' register alias for x18

On Fri, May 15, 2020 at 06:27:53PM +0100, Will Deacon wrote:
> x18 holds the SCS stack pointer value, so introduce a register alias to
> make this easier to read in assembly code.
> 
> Signed-off-by: Will Deacon <will@...nel.org>

I scanned through arm64 for all instances of x18, and it looks like
you've covered all the relevant uses here. In kvm we save/restore x18 a
bunch becasue it might be a platform register, but we do that
unconditionally and without knowledge of what it contains, so I think
that's fine to leave as-is. Therefore:

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

As an aside, the comment in entry-ftrace.S is now stale where it says
that x18 is safe to clobber. I can send a patch to clean that up, unless
you want to do that yourself.

Mark.

> ---
>  arch/arm64/include/asm/scs.h |  6 ++++--
>  arch/arm64/kernel/entry.S    | 10 +++++-----
>  arch/arm64/kernel/head.S     |  2 +-
>  3 files changed, 10 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/scs.h b/arch/arm64/include/asm/scs.h
> index 6b8cf4352fe3..d46efdd2060a 100644
> --- a/arch/arm64/include/asm/scs.h
> +++ b/arch/arm64/include/asm/scs.h
> @@ -7,12 +7,14 @@
>  #include <asm/asm-offsets.h>
>  
>  #ifdef CONFIG_SHADOW_CALL_STACK
> +	scs_sp	.req	x18
> +
>  	.macro scs_load tsk, tmp
> -	ldr	x18, [\tsk, #TSK_TI_SCS_SP]
> +	ldr	scs_sp, [\tsk, #TSK_TI_SCS_SP]
>  	.endm
>  
>  	.macro scs_save tsk, tmp
> -	str	x18, [\tsk, #TSK_TI_SCS_SP]
> +	str	scs_sp, [\tsk, #TSK_TI_SCS_SP]
>  	.endm
>  #else
>  	.macro scs_load tsk, tmp
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index cb0516e6f963..741faf0706f1 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -394,7 +394,7 @@ alternative_insn eret, nop, ARM64_UNMAP_KERNEL_AT_EL0
>  	.macro	irq_stack_entry
>  	mov	x19, sp			// preserve the original sp
>  #ifdef CONFIG_SHADOW_CALL_STACK
> -	mov	x24, x18		// preserve the original shadow stack
> +	mov	x24, scs_sp		// preserve the original shadow stack
>  #endif
>  
>  	/*
> @@ -416,7 +416,7 @@ alternative_insn eret, nop, ARM64_UNMAP_KERNEL_AT_EL0
>  
>  #ifdef CONFIG_SHADOW_CALL_STACK
>  	/* also switch to the irq shadow stack */
> -	adr_this_cpu x18, irq_shadow_call_stack, x26
> +	adr_this_cpu scs_sp, irq_shadow_call_stack, x26
>  #endif
>  
>  9998:
> @@ -430,7 +430,7 @@ alternative_insn eret, nop, ARM64_UNMAP_KERNEL_AT_EL0
>  	.macro	irq_stack_exit
>  	mov	sp, x19
>  #ifdef CONFIG_SHADOW_CALL_STACK
> -	mov	x18, x24
> +	mov	scs_sp, x24
>  #endif
>  	.endm
>  
> @@ -1071,9 +1071,9 @@ SYM_CODE_START(__sdei_asm_handler)
>  #ifdef CONFIG_SHADOW_CALL_STACK
>  	/* Use a separate shadow call stack for normal and critical events */
>  	cbnz	w4, 3f
> -	adr_this_cpu dst=x18, sym=sdei_shadow_call_stack_normal, tmp=x6
> +	adr_this_cpu dst=scs_sp, sym=sdei_shadow_call_stack_normal, tmp=x6
>  	b	4f
> -3:	adr_this_cpu dst=x18, sym=sdei_shadow_call_stack_critical, tmp=x6
> +3:	adr_this_cpu dst=scs_sp, sym=sdei_shadow_call_stack_critical, tmp=x6
>  4:
>  #endif
>  
> diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
> index 2b01c19c5483..1293baddfd20 100644
> --- a/arch/arm64/kernel/head.S
> +++ b/arch/arm64/kernel/head.S
> @@ -426,7 +426,7 @@ SYM_FUNC_START_LOCAL(__primary_switched)
>  	mov	x29, sp
>  
>  #ifdef CONFIG_SHADOW_CALL_STACK
> -	adr_l	x18, init_shadow_call_stack	// Set shadow call stack
> +	adr_l	scs_sp, init_shadow_call_stack	// Set shadow call stack
>  #endif
>  
>  	str_l	x21, __fdt_pointer, x5		// Save FDT pointer
> -- 
> 2.26.2.761.g0e0b3e54be-goog
>