Instead of just telling GCC that dcbz(), dcbi(), dcbf() and dcbst()
clobber memory, tell it what it clobbers:
* dcbz(), dcbi() and dcbf() clobbers one cacheline as output
* dcbf() and dcbst() clobbers one cacheline as input
Signed-off-by: Christophe Leroy <redacted>
---
arch/powerpc/include/asm/cache.h | 17 +++++++++++++----
1 file changed, 13 insertions(+), 4 deletions(-)
Segher,
A while ago I proposed the following patch, and didn't get any comment
back on it.
Do you have any opinion on it ? Is it good and worth it ?
Thanks
Christophe
Le 09/01/2018 à 07:57, Christophe Leroy a écrit :
quoted hunk
Instead of just telling GCC that dcbz(), dcbi(), dcbf() and dcbst()
clobber memory, tell it what it clobbers:
* dcbz(), dcbi() and dcbf() clobbers one cacheline as output
* dcbf() and dcbst() clobbers one cacheline as input
Signed-off-by: Christophe Leroy <redacted>
---
arch/powerpc/include/asm/cache.h | 17 +++++++++++++----
1 file changed, 13 insertions(+), 4 deletions(-)
Segher,
A while ago I proposed the following patch, and didn't get any comment
back on it.
Do you have any opinion on it ? Is it good and worth it ?
Thanks
Christophe
Le 09/01/2018 à 07:57, Christophe Leroy a écrit :
quoted hunk
Instead of just telling GCC that dcbz(), dcbi(), dcbf() and dcbst()
clobber memory, tell it what it clobbers:
* dcbz(), dcbi() and dcbf() clobbers one cacheline as output
* dcbf() and dcbst() clobbers one cacheline as input
Signed-off-by: Christophe Leroy <redacted>
---
arch/powerpc/include/asm/cache.h | 17 +++++++++++++----
1 file changed, 13 insertions(+), 4 deletions(-)
diff --git a/arch/powerpc/include/asm/cache.h
b/arch/powerpc/include/asm/cache.h
index c1d257aa4c2d..fc8fe18acf8c 100644
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 :-)
Do you have any opinion on it ? Is it good and worth it ?
Le 09/01/2018 à 07:57, Christophe Leroy a écrit :
quoted
Instead of just telling GCC that dcbz(), dcbi(), dcbf() and dcbst()
clobber memory, tell it what it clobbers:
* dcbz(), dcbi() and dcbf() clobbers one cacheline as output
* dcbf() and dcbst() clobbers one cacheline as input
You cannot "clobber input".
Seen another way, only dcbi clobbers anything; dcbz zeroes it instead,
and dcbf and dcbst only change in what caches the data hangs out.
The instruction does *not* work on the memory pointed to by addr. It
works on the cache line containing the address addr.
If you want to have addr always aligned, you need to document this, and
check all callers, etc.
Newline damage... Was that your mailer?
Also, you may want a "memory" clobber anyway, to get ordering correct
for the synchronisation instructions.
I think your changes make things less robust than they were before.
[ 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. ]
Segher
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:
quoted
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