Re: [PATCH] powerpc: Add POWER9 copy_page() loop

[Anton Blanchard <anton@samba.org>]

Hi Nick,

> I've got a patch that makes alternate feature patching a bit
> more flexible and not hit relocation limits when using big "else"
> parts. I was thinking of doing something like
> 
> _GLOBAL_TOC(copy_page)
> BEGIN_FTR_SECTION_NESTED(50)
> #include "copypage_power9.S"
> FTR_SECTION_ELSE_NESTED(50)
> #include "copypage_power7.S"
> ALT_FTR_SECTION_END_NESTED_IFSET(CPU_FTR_ARCH_300, 50)

Good idea, I hadn't thought of embedding it all in a feature section.

> I guess POWER asm doesn't need this but it's good practice to prevent
> copy paste errors? It would be nice to have some macros to hide all
> these constants, but that's for another patch. The commenting is good.

The .machine X macros? Unfortunately the format of dcbt is different
for recent server chips. This wasn't a great idea in retrospect because
if you do get the instruction layout wrong, you wont get a fault to warn
you.

> I don't suppose the stream setup is costly enough to consider
> touching a cacheline or two ahead before starting it?

Starting up software streams is a bit of an art - if the demand loads
get ahead then a hardware stream gets started before the software one.
Note all the eieios to try and avoid this happening.

I've struggled with software prefetch on previous chips and sometimes I
wonder if it is worth the pain.

> (Also for another day) We might be able to avoid the stack and call
> for some common cases. Pretty small overcall cost I guess, but it
> could be beneficial for memcpy if not copy_page.

Definitely. Also the breakpoint for using vector should be much
lower if we have already saved the user state in a previous call.

Anton