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

[Laurent Dufour <ldufour@linux.ibm.com>]

Le 20/07/2020 à 22:24, Segher Boessenkool a écrit :
> On Mon, Jul 20, 2020 at 03:10:41PM -0500, Segher Boessenkool wrote:
>> 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.

Oh nice, I was not aware that compiler may choose registers this way.
Good to know, thanks for the explanation.

>>> 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.
> 
> (The explicit register number part, I mean; operand 2 should be an
> output as well, yes.)

Sure if the compiler can choose the registers that's far better.

Cheers,
Laurent.