Re: [PATCH v5 1/7] s390/percpu: Infrastructure for more efficient this_cpu operations

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

On Tue, Jun 02, 2026 at 02:32:09PM +0100, David Laight wrote:
> On Mon, 1 Jun 2026 17:08:13 +0200
> Heiko Carstens <hca@linux.ibm.com> wrote:
> > It is: the check makes sure this is an AG instruction, which adds the
> > percpu offset from lowcore - by checking that the displacement is
> > correct, as well as that the base register is zero.
> > 
> > There could be a different AG instruction within the inline assembly,
> > for whatever reason.
> 
> Do you actually even need to check the instruction?
> 
> This sequence can only work for simple per-cpu accesses, so I don't
> see a reason to let the specified register point anywhere other than the
> base of the per-cpu data.
> 
> That means the process switch code can just load the register with the
> base of the per-cpu data for the new cpu.
> If that happens before the 'AG' is executed it won't matter.
> 
> The only reason would be to support non-offsettable memory accesses.
> But it looks like the 'laag    %r5,%r2,0(%r4)' in the example has an
> offset (of zero).
> Probably only stops you doing a direct access of an array.
> 
> That would mean that needs_fixup goes in the bin and percpu_exit() becomes:
> 	...
> 	reg = regs->percpu_register;
> 	if (likely(!reg))
> 		return;
> 	lc->percpu_register = reg;
> 	regs->gprs[reg] = lc->percpu_offset
> }
> 
> I guess I'm missing something?

The percpu register (in the above example %r4) first contains the base address
of a percpu variable. To get the actual percpu address of the variable the
percpu_offset of the corresponding cpu has to be added to that address, which
is what the AG instruction is doing.

What you propose would make a CPU's percpu_offset the address of any percpu
variable, which most likely would result in a crash.