Am 05.06.24 um 15:37 schrieb Thomas Huth:
On 29/05/2024 17.43, wrote:
From: Jared Rossi <>

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 <>
  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)
+    if (load_next_iplb()) {
+        sclp_print("\nTrying next boot device...");
+        jump_to_IPL_code((long)_start);
+    }

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.

We jump back to _start and to me it looks like that this code does the 
resetting of bss segment.
So anything that has a zero value this should be fine. But static variables != 
0 are indeed tricky.
As far as I can see we do have some of those :-(

So instead of jumping, is there a way that remember somewhere at which device 
we are and then trigger a re-ipl to reload the BIOS?

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