Re: [RFC] in-kernel rseq

[Heiko Carstens <hca@linux.ibm.com>]

On Tue, Feb 24, 2026 at 05:02:10PM +0100, Heiko Carstens wrote:
> 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:

...[random junk]...

Sorry, that was an incorrect version, only handling this_cpu_add().

So with

static DEFINE_PER_CPU(long, bar);

long foo(long val)
{
	return this_cpu_add_return(bar, val);
}

and the above define the result would be the below (no stack frame -
up to the thunk to handle that, including register save/restore).

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 02 10 00 00 e8       laag    %r0,%r2,0(%r1)
  22:   eb ff 03 a8 00 6e       alsi    936,-1
  28:   a7 a4 00 05             jhe     32 <foo+0x32>
  2c:   b9 08 00 20             agr     %r2,%r0
  30:   07 fe                   br      %r14
  32:   e3 e0 f0 88 00 24       stg     %r14,136(%r15)
  38:   c0 e5 00 00 00 00       brasl   %r14,38 <foo+0x38>
                        3a: R_390_PC32DBL       preempt_schedule_notrace_thunk+0x2
  3e:   e3 e0 f0 88 00 04       lg      %r14,136(%r15)
  44:   b9 08 00 20             agr     %r2,%r0
  48:   07 fe                   br      %r14