Re: [PATCH] powerpc/32: Remove memory clobber asm constraint on dcbX() functions

Christophe Leroy
2019-05-06T16:31:47Z

Hi Segher,

On 05/03/2019 06:15 PM, Segher Boessenkool wrote:
> Hi Christophe,
>
> On Fri, May 03, 2019 at 04:14:13PM +0200, Christophe Leroy wrote:
>> A while ago I proposed the following patch, and didn't get any comment
>> back on it.
>
> I didn't see it. Maybe because of holiday :-)

Thanks for this answer, I guess I'll drop it for the time being.

However, I've tried your suggestion below and get unnexpected result.

[...]

>
>
> [ Btw. Instead of
>
> __asm__ __volatile__ ("dcbf 0, %0" : : "r"(addr) : "memory");
>
> you can do
>
> __asm__ __volatile__ ("dcbf %0" : : "Z"(addr) : "memory");
>
> to save some insns here and there. ]

Tried that change on dcbz() and checked function clear_page()

static inline void clear_page(void *addr)
{
unsigned int i;

for (i = 0; i < PAGE_SIZE / L1_CACHE_BYTES; i++, addr += L1_CACHE_BYTES)
dcbz(addr);
}

void clear_user_page(void *page, unsigned long vaddr, struct page *pg)
{
clear_page(page);

/*
* We shouldn't have to do this, but some versions of glibc
* require it (ld.so assumes zero filled pages are icache clean)
* - Anton
*/
flush_dcache_page(pg);
}
EXPORT_SYMBOL(clear_user_page);

Before the change,

clear_user_page:
mflr 0
stw 0,4(1)
bl _mcount
stwu 1,-16(1)
li 9,128
mflr 0
mtctr 9
stw 0,20(1)
.L46:
#APP
# 88 "./arch/powerpc/include/asm/cache.h" 1
dcbz 0, 3
# 0 "" 2
#NO_APP
addi 3,3,32
bdnz .L46
lwz 0,20(1)
mr 3,5
mtlr 0
addi 1,1,16
b flush_dcache_page

After the change

clear_user_page:
mflr 0
stw 0,4(1)
bl _mcount
stwu 1,-32(1)
li 9,128
mflr 0
mtctr 9
stw 0,36(1)
.L46:
stw 3,8(1)
addi 9,1,8
#APP
# 88 "./arch/powerpc/include/asm/cache.h" 1
dcbz 0(9)
# 0 "" 2
#NO_APP
addi 3,3,32
bdnz .L46
mr 3,5
bl flush_dcache_page
lwz 0,36(1)
addi 1,1,32
mtlr 0
blr

So first of all it uses an unexisting form of dcbz : "dcbz 0(9)"
And in addition, it stores r3 somewhere and I guess expects to read it
with dcbz ???

Looks like 'Z' is not the right constraint to use.

Christophe