Re: [PATCH v2] powerpc: slightly improve cache helpers

On Sun, Jul 21, 2019 at 07:41:40PM -0700, Nathan Chancellor wrote:

Hi Segher,

On Sun, Jul 21, 2019 at 01:01:50PM -0500, Segher Boessenkool wrote:

quoted

On Sun, Jul 21, 2019 at 12:58:46AM -0700, Nathan Chancellor wrote:

quoted

0000017c clear_user_page:
     17c: 94 21 ff f0                  	stwu 1, -16(1)
     180: 38 80 00 80                  	li 4, 128
     184: 38 63 ff e0                  	addi 3, 3, -32
     188: 7c 89 03 a6                  	mtctr 4
     18c: 38 81 00 0f                  	addi 4, 1, 15
     190: 8c c3 00 20                  	lbzu 6, 32(3)
     194: 98 c1 00 0f                  	stb 6, 15(1)
     198: 7c 00 27 ec                  	dcbz 0, 4
     19c: 42 00 ff f4                  	bdnz .+65524

Uh, yeah, well, I have no idea what clang tried here, but that won't
work.  It's copying a byte from each target cache line to the stack,
and then does clears the cache line containing that byte on the stack.

I *guess* this is about "Z" and not about "%y", but you'll have to ask
the clang people.

Or it may be that they do not treat inline asm operands as lvalues
properly?  That rings some bells.  Yeah that looks like it.

The code is
  __asm__ __volatile__ ("dcbz %y0" : : "Z"(*(u8 *)addr) : "memory");

so yeah it looks like clang took that  *(u8 *)addr  as rvalue, and
stored that in stack, and then used *that* as memory.

Maybe clang simply does not not to treat "Z" the same as "m"?  (And "Y"
and "Q" and "es" and a whole bunch of "w*", what about those?)


Segher