linux-kernel - Re: [PATCH 1/3] arm64: adjust el0_sync so that a function can be called

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Date:	Mon, 05 May 2014 15:44:30 -0700
From:	Kevin Hilman <khilman@...aro.org>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	Will Deacon <Will.Deacon@....com>,
	Mark Rutland <mark.rutland@....com>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel\@lists.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linaro-kernel\@lists.linaro.org" <linaro-kernel@...ts.linaro.org>,
	"khilman\@linaro.org" <khilman@...aro.org>
Subject: Re: [PATCH 1/3] arm64: adjust el0_sync so that a function can be called

[+ Mark R. since we looked at this together a little at ELC]

Catalin Marinas <catalin.marinas@....com> writes:

> On Sun, Apr 27, 2014 at 08:44:12PM +0100, Larry Bassel wrote:
>> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
>> index 39ac630..eda7755 100644
>> --- a/arch/arm64/kernel/entry.S
>> +++ b/arch/arm64/kernel/entry.S
> [...]
>> @@ -421,28 +421,30 @@ el0_da:
>>  	/*
>>  	 * Data abort handling
>>  	 */
>> -	mrs	x0, far_el1
>> -	bic	x0, x0, #(0xff << 56)
>>  	disable_step x1
>>  	isb
>>  	enable_dbg
>>  	// enable interrupts before calling the main handler
>>  	enable_irq
>> +	mrs	x0, far_el1
>> +	bic	x0, x0, #(0xff << 56)
>>  	mov	x1, x25
>>  	mov	x2, sp

>>  	b	do_mem_abort
>
> Reading the far_el1 after enable_dbg and enable_irq is racy, we can no
> longer guarantee its value in the original data abort context.

Catalin, can you confirm x26 would be a safe place to stash far_el1 and
then restore it after ct_user_exit.  Something like this (which seems to
work for me):

el0_da:
        /*                                                                                                                                                    * Data abort handling                                                                                                                                */
        mrs     x26, far_el1
        disable_step x1
        isb
        enable_dbg
        // enable interrupts before calling the main handler                                                                                          
        enable_irq
        ct_user_exit
        mov     x0, x26
        bic     x0, x0, #(0xff << 56)
        mov     x1, x25
        mov     x2, sp
        adr     lr, ret_from_exception
        b       do_mem_abort

Kevin
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