linux-kernel - Re: [PATCH v3 2/2] arm64: enable context tracking

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Date:	Mon, 19 May 2014 16:32:20 +0100
From:	Will Deacon <will.deacon@....com>
To:	Larry Bassel <larry.bassel@...aro.org>
Cc:	Catalin Marinas <Catalin.Marinas@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	"khilman@...aro.org" <khilman@...aro.org>
Subject: Re: [PATCH v3 2/2] arm64: enable context tracking

On Fri, May 09, 2014 at 10:51:50PM +0100, Larry Bassel wrote:
> Make calls to ct_user_enter when the kernel is exited
> and ct_user_exit when the kernel is entered (in el0_da,
> el0_ia, el0_svc, el0_irq and all of the "error" paths).
> 
> These macros expand to function calls which will only work
> properly if el0_sync and related code has been rearranged
> (in a previous patch of this series).
> 
> The calls to ct_user_exit are made after hw debugging has been
> enabled (enable_dbg).
> 
> The call to ct_user_enter is made at the beginning of the
> kernel_exit macro.
> 
> This patch is based on earlier work by Kevin Hilman.

[...]

> diff --git a/arch/arm64/include/asm/thread_info.h b/arch/arm64/include/asm/thread_info.h
> index 720e70b..301ea6a 100644
> --- a/arch/arm64/include/asm/thread_info.h
> +++ b/arch/arm64/include/asm/thread_info.h
> @@ -108,6 +108,7 @@ static inline struct thread_info *current_thread_info(void)
>  #define TIF_SINGLESTEP		21
>  #define TIF_32BIT		22	/* 32bit process */
>  #define TIF_SWITCH_MM		23	/* deferred switch_mm */
> +#define TIF_NOHZ                24
>  
>  #define _TIF_SIGPENDING		(1 << TIF_SIGPENDING)
>  #define _TIF_NEED_RESCHED	(1 << TIF_NEED_RESCHED)
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index 136bb7d..c839bab 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -30,6 +30,44 @@
>  #include <asm/unistd32.h>
>  
>  /*
> + * Context tracking subsystem.  Used to instrument transitions
> + * between user and kernel mode.
> + */
> +	.macro ct_user_exit, save = 0
> +#ifdef CONFIG_CONTEXT_TRACKING
> +	bl	context_tracking_user_exit
> +	.if \save == 1

It would be clearer to refer to the parameter as `restore' for the exit
case, I reckon.

> +	/*
> +	 * save/restore needed during syscalls.  Restore syscall arguments from
> +	 * the values already saved on stack during kernel_entry
> +	 */
> +	ldp	x0, x1, [sp]
> +	ldp	x2, x3, [sp, #S_X2]
> +	ldp	x4, x5, [sp, #S_X4]
> +	ldp	x6, x7, [sp, #S_X6]
> +	.endif
> +#endif
> +	.endm

[...]

>  	.macro	kernel_exit, el, ret = 0
>  	ldp	x21, x22, [sp, #S_PC]		// load ELR, SPSR
>  	.if	\el == 0
> +	ct_user_enter \ret
>  	ldr	x23, [sp, #S_SP]		// load return stack pointer
>  	.endif
>  	.if	\ret

You should check how this patch applies against my debug exception rework:

2a2830703a23 ("arm64: debug: avoid accessing mdscr_el1 on fault paths where
possible") in today's next.

As well as addressing the conflicts, it's worth nothing that the thread_info
becomes available in tsk much earlier for exceptions from userspace with
that patch applied.

Will
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