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Date: Wed, 28 May 2014 12:27:38 +0100
From: Will Deacon <will.deacon@arm.com>
To: Larry Bassel <larry.bassel@linaro.org>
Cc: Catalin Marinas <Catalin.Marinas@arm.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
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Subject: Re: [PATCH v5 1/2] arm64: adjust el0_sync so that a function can be
 called
Message-ID: <20140528112738.GE15222@arm.com>
References: <1401130573-7443-1-git-send-email-larry.bassel@linaro.org>
 <1401130573-7443-2-git-send-email-larry.bassel@linaro.org>
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Hi Larry,

On Mon, May 26, 2014 at 07:56:12PM +0100, Larry Bassel wrote:
> To implement the context tracker properly on arm64,
> a function call needs to be made after debugging and
> interrupts are turned on, but before the lr is changed
> to point to ret_to_user(). If the function call
> is made after the lr is changed the function will not
> return to the correct place.
> 
> For similar reasons, defer the setting of x0 so that
> it doesn't need to be saved around the function call
> (save far_el1 in x26 temporarily instead).
> 
> Signed-off-by: Larry Bassel <larry.bassel@linaro.org>

[...]

> @@ -476,23 +481,27 @@ el0_undef:
>  	// enable interrupts before calling the main handler
>  	enable_dbg_and_irq
>  	mov	x0, sp
> +	adr	lr, ret_to_user
>  	b	do_undefinstr
>  el0_dbg:
>  	/*
>  	 * Debug exception handling
>  	 */
>  	tbnz	x24, #0, el0_inv		// EL0 only
> -	mrs	x0, far_el1
> +	mrs	x26, far_el1
> +	mov	x0, x26
>  	mov	x1, x25
>  	mov	x2, sp
>  	bl	do_debug_exception
>  	enable_dbg
> +	mov	x0, x26
>  	b	ret_to_user

Why have you added this mov instruction?

Will