Re: [kvm-unit-tests RFC 2/2] lib: s390x: css: Name inline assembly arguments and clean them up

Heiko Carstens
2024-04-22T10:57:30Z

On Mon, Apr 22, 2024 at 09:43:44AM +0200, Nico Boehr wrote:
> Quoting Janosch Frank (2024-02-01 15:23:56)
> [...]
> > diff --git a/lib/s390x/css.h b/lib/s390x/css.h
> > index 504b3f14..e4311124 100644
> > --- a/lib/s390x/css.h
> > +++ b/lib/s390x/css.h
> [...]
> > @@ -167,11 +167,11 @@ static inline int msch(unsigned long schid, struct schib *addr)
> > int cc;
> >
> > asm volatile(
> > - " msch 0(%3)\n"
> > - " ipm %0\n"
> > - " srl %0,28"
> > - : "=d" (cc)
> > - : "d" (reg1), "m" (*addr), "a" (addr)
> > + " msch 0(%[addr])\n"
> > + " ipm %[cc]\n"
> > + " srl %[cc],28"
> > + : [cc] "=d" (cc)
> > + : "d" (reg1), [addr] "a" (addr)
>
> I think there was a reason why the "m"(*addr) was here. Either add it back
> or add a memory clobber.

It is there to tell the compiler that the memory contents at *addr are used
as input. Without that, and only the "a" contraint, the compiler is free to
discard any potential previous writes to *addr.

The best solution here would be to use the Q constraint (memory reference
with short displacement and without index register) for the second operand
address of msch. Or simply copy the current implementation from the kernel
(drivers/s390/cio/ioasm.c).