dcbz not used on ppc64?

dcbz not used on ppc64?

[Chris Friesen]

I ran into some issues with the use of the dcbz instruction in the 
kernel for ppc32.  I'm now looking at porting our work to ppc64, and 
when I looked around a bit, I found that the dcbz instruction doesn't 
appear to be used in ppc64.

This leads me to ask:

1) Is it in fact true that ppc64 does not use dcbz in the kernel?
2) If so, why not?  Presumably it would be a win in some cases, 
otherwise they wouldn't be using it in ppc32.  Is there some other 
factore in play for ppc64?

Thanks,

Chris

Re: dcbz not used on ppc64?

[Anton Blanchard]

 
Hi,

> I ran into some issues with the use of the dcbz instruction in the 
> kernel for ppc32.  I'm now looking at porting our work to ppc64, and 
> when I looked around a bit, I found that the dcbz instruction doesn't 
> appear to be used in ppc64.
> 
> This leads me to ask:
> 
> 1) Is it in fact true that ppc64 does not use dcbz in the kernel?
> 2) If so, why not?  Presumably it would be a win in some cases, 
> otherwise they wouldn't be using it in ppc32.  Is there some other 
> factore in play for ppc64?

include/asm-ppc64/page.h:clear_page() uses it:

static __inline__ void clear_page(void *addr)
{
        unsigned long lines, line_size;

        line_size = systemcfg->dCacheL1LineSize;
        lines = naca->dCacheL1LinesPerPage;

        __asm__ __volatile__(
        "mtctr          %1      # clear_page\n\
1:      dcbz    0,%0\n\
        add     %0,%0,%3\n\
        bdnz+   1b"
        : "=r" (addr)
        : "r" (lines), "0" (addr), "r" (line_size)
        : "ctr", "memory");
}

Anton

Re: dcbz not used on ppc64?

[Chris Friesen]

Anton Blanchard wrote:

>>1) Is it in fact true that ppc64 does not use dcbz in the kernel?
>>2) If so, why not?  Presumably it would be a win in some cases, 
>>otherwise they wouldn't be using it in ppc32.  Is there some other 
>>factore in play for ppc64?
> 
> 
> include/asm-ppc64/page.h:clear_page() uses it:

Hmm.  As of 2.6.9 at least, clear_page() is only ever called by 
clear_user_page(), which in turn is only ever called by 
drivers/media/video/video-buf.c.  I'm not using that driver, which 
explains why the instruction didn't show up when I disassembled the kernel.

Still, ppc32 also uses it in cacheable_memzero and __copy_tofrom_user. 
It's also used in cacheable_memcpy, but that routine doesn't seem to be 
used anywhere.

Chris

Re: dcbz not used on ppc64?

[Anton Blanchard]

 
> Hmm.  As of 2.6.9 at least, clear_page() is only ever called by 
> clear_user_page(), which in turn is only ever called by 
> drivers/media/video/video-buf.c.  I'm not using that driver, which 
> explains why the instruction didn't show up when I disassembled the kernel.

Dont forget mm/memory.c:clear_user_highpage -> clear_user_page. 

Anton