linux-kernel - Re: [PATCH 1/5] watchdog: diag288

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Date:   Fri, 3 Feb 2023 10:17:28 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Alexander Egorenkov <egorenar@...ux.ibm.com>
Cc:     wim@...ux-watchdog.org, linux-watchdog@...r.kernel.org,
        linux-kernel@...r.kernel.org, hca@...ux.ibm.com
Subject: Re: [PATCH 1/5] watchdog: diag288_wdt: get rid of register asm

On Fri, Feb 03, 2023 at 08:39:54AM +0100, Alexander Egorenkov wrote:
> Using register asm statements has been proven to be very error prone,
> especially when using code instrumentation where gcc may add function
> calls, which clobbers register contents in an unexpected way.
> 
> Therefore, get rid of register asm statements in watchdog code, and make
> sure this bug class cannot happen.
> 
> Moreover, remove the register r1 from the clobber list because this
> register is not changed by DIAG 288.
> 
> Reviewed-by: Heiko Carstens <hca@...ux.ibm.com>
> Signed-off-by: Alexander Egorenkov <egorenar@...ux.ibm.com>

Reviewed-by: Guenter Roeck <linux@...ck-us.net>

> ---
>  drivers/watchdog/diag288_wdt.c | 15 +++++++--------
>  1 file changed, 7 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/watchdog/diag288_wdt.c b/drivers/watchdog/diag288_wdt.c
> index 6ca5d9515d85..07ebbb709af4 100644
> --- a/drivers/watchdog/diag288_wdt.c
> +++ b/drivers/watchdog/diag288_wdt.c
> @@ -73,20 +73,19 @@ MODULE_ALIAS("vmwatchdog");
>  static int __diag288(unsigned int func, unsigned int timeout,
>  		     unsigned long action, unsigned int len)
>  {
> -	register unsigned long __func asm("2") = func;
> -	register unsigned long __timeout asm("3") = timeout;
> -	register unsigned long __action asm("4") = action;
> -	register unsigned long __len asm("5") = len;
> +	union register_pair r1 = { .even = func, .odd = timeout, };
> +	union register_pair r3 = { .even = action, .odd = len, };
>  	int err;
>  
>  	err = -EINVAL;
>  	asm volatile(
> -		"	diag	%1, %3, 0x288\n"
> -		"0:	la	%0, 0\n"
> +		"	diag	%[r1],%[r3],0x288\n"
> +		"0:	la	%[err],0\n"
>  		"1:\n"
>  		EX_TABLE(0b, 1b)
> -		: "+d" (err) : "d"(__func), "d"(__timeout),
> -		  "d"(__action), "d"(__len) : "1", "cc", "memory");
> +		: [err] "+d" (err)
> +		: [r1] "d" (r1.pair), [r3] "d" (r3.pair)
> +		: "cc", "memory");
>  	return err;
>  }
>