Re: [PATCH v2] pc-bios/s390-ccw: Use reset_psw pointer instead of hard-coded null pointer

[Janosch Frank]

On 4/23/21 4:24 PM, Thomas Huth wrote:
> When compiling the s390-ccw bios with clang, it emits a warning like this:
> 
>  pc-bios/s390-ccw/jump2ipl.c:86:9: warning: indirection of non-volatile null
>   pointer will be deleted, not trap [-Wnull-dereference]
>      if (*((uint64_t *)0) & RESET_PSW_MASK) {
>          ^~~~~~~~~~~~~~~~
>  pc-bios/s390-ccw/jump2ipl.c:86:9: note: consider using __builtin_trap() or
>   qualifying pointer with 'volatile'
> 
> We could add a "volatile" here to shut it up, but on the other hand,
> we also have a pointer variable called "reset_psw" in this file already
> that points to the PSW at address 0, so we can simply use that pointer
> variable instead.

LGTM, I'm just wondering why I didn't clean that up when I last changed
the file.

Reviewed-by: Janosch Frank <frankja@linux.ibm.com>

> 
> Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
> Signed-off-by: Thomas Huth <thuth@redhat.com>
> ---
>  v2: Extend comment as suggested by Cornelia
> 
>  pc-bios/s390-ccw/jump2ipl.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/pc-bios/s390-ccw/jump2ipl.c b/pc-bios/s390-ccw/jump2ipl.c
> index b9c70d64a5..73e4367e09 100644
> --- a/pc-bios/s390-ccw/jump2ipl.c
> +++ b/pc-bios/s390-ccw/jump2ipl.c
> @@ -82,8 +82,8 @@ void jump_to_low_kernel(void)
>          jump_to_IPL_code(KERN_IMAGE_START);
>      }
>  
> -    /* Trying to get PSW at zero address */
> -    if (*((uint64_t *)0) & RESET_PSW_MASK) {
> +    /* Trying to get PSW at zero address (pointed to by reset_psw) */
> +    if (*reset_psw & RESET_PSW_MASK) {
>          /*
>           * Surely nobody will try running directly from lowcore, so
>           * let's use 0 as an indication that we want to load the reset
>