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SubjectRe: [PATCH v2] powerpc: slightly improve cache helpers
On Sun, Jul 21, 2019 at 11:19 PM Segher Boessenkool
<segher@kernel.crashing.org> wrote:
>
> 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:
> > > On Sun, Jul 21, 2019 at 12:58:46AM -0700, Nathan Chancellor wrote:
> > > > 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.

What's the %y modifier supposed to mean here? addr is in the list of
inputs, so what's wrong with using it as an rvalue?

>
> 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?)

--
Thanks,
~Nick Desaulniers

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