[RFC PATCH] powerpc/pseries/svm: capture instruction faulting on MMIO access, in sprg0 register

[Segher Boessenkool]

On Mon, Jul 20, 2020 at 11:39:56AM +0200, Laurent Dufour wrote:
> Le 16/07/2020 à 10:32, Ram Pai a écrit :
> >+	if (is_secure_guest()) {					\
> >+		__asm__ __volatile__("mfsprg0 %3;"			\
> >+				"lnia %2;"				\
> >+				"ld %2,12(%2);"				\
> >+				"mtsprg0 %2;"				\
> >+				"sync;"					\
> >+				#insn" %0,%y1;"				\
> >+				"twi 0,%0,0;"				\
> >+				"isync;"				\
> >+				"mtsprg0 %3"				\
> >+			: "=r" (ret)					\
> >+			: "Z" (*addr), "r" (0), "r" (0)			\
> 
> I'm wondering if SPRG0 is restored to its original value.
> You're using the same register (r0) for parameters 2 and 3, so when doing 
> lnia %2, you're overwriting the SPRG0 value you saved in r0 just earlier.

It is putting the value 0 in the registers the compiler chooses for
operands 2 and 3.  But operand 3 is written, while the asm says it is an
input.  It needs an earlyclobber as well.

> It may be clearer to use explicit registers for %2 and %3 and to mark them 
> as modified for the compiler.

That is not a good idea, imnsho.


Segher