linux-kernel - Re: [RFC PATCH 1/3] arm64: entry: Remove unnecessary calculation for S

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Date:	Mon, 07 Sep 2015 15:48:49 +0100
From:	James Morse <james.morse@....com>
To:	Jungseok Lee <jungseoklee85@...il.com>
CC:	Catalin Marinas <Catalin.Marinas@....com>,
	Will Deacon <Will.Deacon@....com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 1/3] arm64: entry: Remove unnecessary calculation
 for S_SP in EL1h

On 04/09/15 15:23, Jungseok Lee wrote:
> Under EL1h, S_SP data is not seen in kernel_exit. Thus, x21 calculation
> is not needed in kernel_entry. Currently, S_SP information is vaild only
> when sp_el0 is used.
> 
> Signed-off-by: Jungseok Lee <jungseoklee85@...il.com>
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index e163518..d23ca0d 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -91,8 +91,6 @@
>  	get_thread_info tsk			// Ensure MDSCR_EL1.SS is clear,
>  	ldr	x19, [tsk, #TI_FLAGS]		// since we can unmask debug
>  	disable_step_tsk x19, x20		// exceptions when scheduling.
> -	.else
> -	add	x21, sp, #S_FRAME_SIZE
>  	.endif
>  	mrs	x22, elr_el1
>  	mrs	x23, spsr_el1
> 

This sp value gets written to the struct pt_regs that is built on the
stack, and passed to the fault handlers, see 'el1_sp_pc' in kernel/entry.S,
which goes on to call do_sp_pc_abort() which prints this value out. (Other
fault handlers may make decisions based on this value).
It should be present and correct.


Thanks,

James
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