[PATCH] powerpc: unaligned memcpy and DMA

[OndÅej BÃlka <neleai at seznam dot cz>]

On Tue, Jan 06, 2015 at 05:12:01PM -0200, Adhemerval Zanella wrote:
> On 06-01-2015 16:53, OndÅej BÃlka wrote:
> >
> > Main question is why there is no power8 memcpy using unaligned loads yet?
> >
> > Memcpy is called about hundred times more often than strcpy(and no
> > strncpy call) on my computer so possible gains are bigger and with 
> > optimized memcpy a generic strncpy will be faster as well.
> 
> Mainly because powerpc still triggers kernel traps when issuing VMX/VSX instruction
> on non-cacheable memory. That's why I pushed 87868c2418fb74357757e3b739ce5b76b17a8929
> by the way.
> 
> Although it is not really an issue for 99% of cases, where memory will be cacheable;
> some code (specially libdrm and xorg), uses memcpy (and possible memset) on DMA mapped
> memory.  And that's why memcpy/memset for POWER8 are still using aligned accesses all
> 5b76b17a8929

That looks like overkill. Better way would be add variable that detects
if application can do it.

A probably simplest way would be add variable in vdso that kernel sets
to 1 when doing trap.

Otherwise it would be more complicated as we would need set it when
application allocates noncachable memory, is mmap only way to do that?