Re: [PATCH 5/5] s390x: Enable and document boot device fallback on panic

[Thomas Huth]

On 29/05/2024 17.43, jrossi@linux.ibm.com wrote:
> From: Jared Rossi <jrossi@linux.ibm.com>
>
> On a panic during IPL (i.e. a device failed to boot) check for another device
> to boot from, as indicated by the presence of an unused IPLB.
>
> If an IPLB is successfully loaded, then jump to the start of BIOS, restarting
> IPL using the updated IPLB.  Otherwise enter disabled wait.
>
> Signed-off-by: Jared Rossi <jrossi@linux.ibm.com>
> ---
>   docs/system/bootindex.rst         | 7 ++++---
>   docs/system/s390x/bootdevices.rst | 9 ++++++---
>   pc-bios/s390-ccw/s390-ccw.h       | 6 ++++++
>   3 files changed, 16 insertions(+), 6 deletions(-)

Could you please split the documentation changes into a separate patch in v2 
? ... I think that would be cleaner.

> diff --git a/docs/system/bootindex.rst b/docs/system/bootindex.rst
> index 8b057f812f..de597561bd 100644
> --- a/docs/system/bootindex.rst
> +++ b/docs/system/bootindex.rst
> @@ -50,9 +50,10 @@ Limitations
>   
>   Some firmware has limitations on which devices can be considered for
>   booting.  For instance, the PC BIOS boot specification allows only one
> -disk to be bootable.  If boot from disk fails for some reason, the BIOS
> -won't retry booting from other disk.  It can still try to boot from
> -floppy or net, though.
> +disk to be bootable, except for on s390x machines. If boot from disk fails for
> +some reason, the BIOS won't retry booting from other disk.  It can still try to
> +boot from floppy or net, though.  In the case of s390x, the BIOS will try up to
> +8 total devices, any number of which may be disks.

Since the old text was already talking about "PC BIOS", I'd rather leave 
that paragraph as it is (maybe just replace "PC BIOS" with "x86 PC BIOS"), 
and add a separate paragraph afterwards about s390x instead.

> diff --git a/pc-bios/s390-ccw/s390-ccw.h b/pc-bios/s390-ccw/s390-ccw.h
> index c977a52b50..de3d1f0d5a 100644
> --- a/pc-bios/s390-ccw/s390-ccw.h
> +++ b/pc-bios/s390-ccw/s390-ccw.h
> @@ -43,6 +43,7 @@ typedef unsigned long long u64;
>   #include "iplb.h"
>   
>   /* start.s */
> +extern char _start[];
>   void disabled_wait(void) __attribute__ ((__noreturn__));
>   void consume_sclp_int(void);
>   void consume_io_int(void);
> @@ -88,6 +89,11 @@ __attribute__ ((__noreturn__))
>   static inline void panic(const char *string)
>   {
>       sclp_print(string);
> +    if (load_next_iplb()) {
> +        sclp_print("\nTrying next boot device...");
> +        jump_to_IPL_code((long)_start);
> +    }
> +
>       disabled_wait();
>   }

Honestly, I am unsure whether this is a really cool idea or a very ugly hack 
... but I think I tend towards the latter, sorry. Jumping back to the 
startup code might cause various problem, e.g. pre-initialized variables 
don't get their values reset, causing different behavior when the s390-ccw 
bios runs a function a second time this way. Thus this sounds very fragile. 
Could we please try to get things cleaned up correctly, so that functions 
return with error codes instead of panicking when we can continue with 
another boot device? Even if its more work right now, I think this will be 
much more maintainable in the future.

 Thomas