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

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Date:   Mon, 6 May 2019 16:31:38 +0000
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     linux-kernel@...r.kernel.org, Scott Wood <oss@...error.net>,
        Paul Mackerras <paulus@...ba.org>,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] powerpc/32: Remove memory clobber asm constraint on
 dcbX() functions

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