Re: [PATCH] powerpc/vdso64: inline __get_datapage()

[Segher Boessenkool <segher@kernel.crashing.org>]

On Wed, Aug 21, 2019 at 01:50:52PM +0200, Christophe Leroy wrote:
> Le 21/08/2019 à 13:44, Segher Boessenkool a écrit :
> >Calls are cheap, in principle...  It is the LR stuff that can make it
> >slower on some cores, and a lot of calling sequence stuff may have
> >considerable overhead of course.
> 
> On an 8xx, a taken branch is 2 cycles and a non taken branch in 1 cycle 
> (+ the refetch if that was not the anticipate branch).

Yup.  And on the big cores they are all 0 cycles, if correctly predicted.
(Taken branches end your fetch group, of course, there are small
inefficiencies everywhere, but that's about the gist of it).

> >>+.macro get_datapage ptr, tmp
> >>+	bcl	20,31,888f
> >>+888:
> >>+	mflr	\ptr
> >>+	addi	\ptr, \ptr, __kernel_datapage_offset - 888b
> >>+	lwz	\tmp, 0(\ptr)
> >>+	add	\ptr, \tmp, \ptr
> >>+.endm
> >
> >(You can just write that as
> >	bcl 20,31,$+4
> >	mflr \ptr
> >etc.  Useless labels are useless :-) )
> 
> Nice trick. Will use that.

Or .+4 if you like that syntax better...  It's all the same thing.

> >One thing you might want to do to improve performance is to do this without
> >the bcl etc., because you cannot really hide the LR latency of that.  But
> >that isn't very many ns either...  Superscalar helps, OoO helps, but it is
> >mostly just that >100MHz helps ;-)
> 
> Good idea. Did you have a look at my vdso32 similar patch ? 
> https://patchwork.ozlabs.org/patch/1148274/

Yes, I saw it.

> Do you have any idea on how to avoid that bcl/mflr stuff ?

Do a load from some fixed address?  Maybe an absolute address, even?
lwz r3,-12344(0)  or similar (that address is in kernel space...)

There aren't many options, and certainly not many *good* options!


Segher