Re: [RFC] in-kernel rseq

[Heiko Carstens]

On Tue, Feb 24, 2026 at 04:20:32PM +0100, Peter Zijlstra wrote:
> > With PREEMPT_LAZY this turns into:
> > 
> > 0000000000000000 <foo>:
> >    0:   c0 04 00 00 00 00       jgnop   0 <foo>
> >    6:   eb af f0 68 00 24       stmg    %r10,%r15,104(%r15)
> >    c:   b9 04 00 ef             lgr     %r14,%r15
> >   10:   b9 04 00 b2             lgr     %r11,%r2
> >   14:   e3 f0 ff c8 ff 71       lay     %r15,-56(%r15)
> >   1a:   e3 e0 f0 98 00 24       stg     %r14,152(%r15) <- up to here: create stack frame
> 
> So some of that could be elided with that asm call thunk thing we talked
> about yesterday, right?

Yes, with
#define __preempt_schedule_notrace() \
 asm volatile("brasl %%r14,preempt_schedule_notrace_thunk" : : : "cc", "memory", "r14")

we would end up with:

0000000000000000 <foo>:
   0:   c0 04 00 00 00 00       jgnop   0 <foo>
   6:   eb 01 03 a8 00 6a       asi     936,1
   c:   c0 10 00 00 00 00       larl    %r1,c <foo+0xc>
                        e: R_390_PC32DBL        .data..percpu+0x2
  12:   a7 39 00 00             lghi    %r3,0
  16:   e3 10 33 b8 00 08       ag      %r1,952(%r3)
  1c:   eb 22 10 00 00 f8       laa     %r2,%r2,0(%r1)
  22:   eb ff 03 a8 00 6e       alsi    936,-1
  28:   a7 a4 00 03             jhe     2e <foo+0x2e>
  2c:   07 fe                   br      %r14
  2e:   e3 e0 f0 88 00 24       stg     %r14,136(%r15)
  34:   c0 e5 00 00 00 00       brasl   %r14,34 <foo+0x34>
                        36: R_390_PC32DBL       preempt_schedule_notrace_thunk+0x2
  3a:   e3 e0 f0 88 00 04       lg      %r14,136(%r15)
  40:   07 fe                   br      %r14

The stack setup is gone, like wanted :)