Re: [PATCH 2/2] pc-bios: s390x: Give precedence to reset PSW

[Thomas Huth <thuth@redhat.com>]

On 19/11/2020 17.57, Eric Farman wrote:
> Let's look at the Reset PSW first instead of the contents of memory.
> It might be leftover from an earlier system boot when processing
> a chreipl.
> 
> Signed-off-by: Eric Farman <farman@linux.ibm.com>
> ---
>  pc-bios/s390-ccw/jump2ipl.c | 20 ++++++++++----------
>  1 file changed, 10 insertions(+), 10 deletions(-)
> 
> diff --git a/pc-bios/s390-ccw/jump2ipl.c b/pc-bios/s390-ccw/jump2ipl.c
> index fbae45b03c..67b4afb6a0 100644
> --- a/pc-bios/s390-ccw/jump2ipl.c
> +++ b/pc-bios/s390-ccw/jump2ipl.c
> @@ -72,16 +72,6 @@ void jump_to_IPL_code(uint64_t address)
>  
>  void jump_to_low_kernel(void)
>  {
> -    /*
> -     * If it looks like a Linux binary, i.e. there is the "S390EP" magic from
> -     * arch/s390/kernel/head.S here, then let's jump to the well-known Linux
> -     * kernel start address (when jumping to the PSW-at-zero address instead,
> -     * the kernel startup code fails when we booted from a network device).
> -     */
> -    if (!memcmp((char *)0x10008, "S390EP", 6)) {
> -        jump_to_IPL_code(KERN_IMAGE_START);
> -    }
> -
>      /* Trying to get PSW at zero address */
>      if (*((uint64_t *)0) & RESET_PSW_MASK) {
>          /*
> @@ -92,6 +82,16 @@ void jump_to_low_kernel(void)
>          jump_to_IPL_code(0);
>      }
>  
> +    /*
> +     * If it looks like a Linux binary, i.e. there is the "S390EP" magic from
> +     * arch/s390/kernel/head.S here, then let's jump to the well-known Linux
> +     * kernel start address (when jumping to the PSW-at-zero address instead,
> +     * the kernel startup code fails when we booted from a network device).
> +     */
> +    if (!memcmp((char *)0x10008, "S390EP", 6)) {
> +        jump_to_IPL_code(KERN_IMAGE_START);
> +    }

That feels a little bit dangerous ... I assume the order has been that way
for a reason, e.g. I think we had to jump to KERN_IMAGE_START for some older
versions of the Linux kernel since the startup code that was referenced by
the PSW at address zero was not working in KVM...

What do you think about this alternate idea instead: Clear the memory at
location 0x10008 at the very beginning of the main() function (or maybe in
boot_setup())? Then we can be sure that there is no stale S390EP magic
dangling around anymore once we've loaded the new kernel...

 Thomas