Re: [PATCH v2 03/10] riscv: Implement cmpxchg8/16() using Zabha

[Andrea Parri <parri.andrea@gmail.com>]

> > I admit that I found this all quite difficult to read; IIUC, this is
> > missing an IS_ENABLED(CONFIG_RISCV_ISA_ZACAS) check.
> 
> I'm not sure we need the zacas check here, since we could use a toolchain
> that supports zabha but not zacas, run this on a zabha/zacas platform and it
> would work.

One specific set-up I was concerned about is as follows:

  1) hardware implements both zabha and zacas
  2) toolchain supports both zabha and zacas
  3) CONFIG_RISCV_ISA_ZABHA=y and CONFIG_RISCV_ISA_ZACAS=n

Since CONFIG_RISCV_ISA_ZABHA=y, the first asm goto will get executed
and, since the hardware implements zacas, that will result in a nop.
Then the second asm goto will get executed and, since the hardware
implements zabha, it will result in the j zabha.  In conclusion, the
amocas instruction following the zabha: label will get executed, thus
violating (the semantics of) CONFIG_RISCV_ISA_ZACAS=n.  IIUC, the diff
I've posted previously in this thread shared a similar limitation/bug.

  Andrea