On Fri, Jan 30, 2015 at 09:39:41AM +0000, Markus Stockhausen wrote:
> > Von: Gabriel Paubert [paub...@iram.es]
> > Gesendet: Freitag, 30. Januar 2015 09:49
> > An: Markus Stockhausen
> > Cc: Scott Wood; linuxppc-dev@lists.ozlabs.org; Herbert Xu
> > Betreff: Re: AW: SPE & Interrupt context (was how to make use of SPE
> > instructions)
> >
> > > ...
> > > - I must already save several non-volatile registers. Putting the 64 bit
> > > values
> > > into them would require me to save their contents with evstdd instead of
> > > stw. Of course stack alignment to 8 bytes required. So only a few
> > > alignment
> > > instructions needed additionally during initialization.
> >
> > On most PPC ABI the stack is guaranteed to be aligned to a 16 byte
> > boundary. In some it may be only 8, but I can't remember any 4 byte
> > only alignment.
> >
> > I checked my 32 bit kernel images with:
> >
> > objdump -d vmlinux |awk '/stwu.*r1,/{print $6,$7}'|sort -u
> >
> > and the stack seems to always be 16 byte aligned.
> > For 64 bit, use stdu instead of stwu.
> >
> > I've also found a few stwux/stdux which are hopefully known
> > to be harmless.
> >
> > Gabriel
>
> A helpful annotation. But now I'm unsure about function usage. SPE seems to be
> 32bit only and I would use their evxxx instructions. Do you think the
> following
> sequence will be the right way?
>
> _GLOBAL(ppc_spe_sha256_transform)
> stwu r1,-128(r1); /* create stack frame */
> stw r24,8(r1); /* save normal registers */
> stw r25,12(r1);
> evstdw r14,16(r1); /* We must save non volatile */
> evstdw r15,24(r1); /* registers. Take the chance */
> evstdw r16,32(r12); /* and save the SPE part too */ \
> ...
> lwz r24,8(r1); /* restore normal registers */ \
> lwz r25,12(r1);
> evldw r14,16(r12); /* restore non-v. + SPE registers */
> evldw r15,24(r12);
> evldw r16,32(r12);
> addi r1,r1,128; /* cleanup stack frame */
>
Yes. But there is also probably a status/control register somewhere that
you might need to save restore, unless it is never used and/or affected by the
instructions you use.
> Or must I use the kernel provided defines with PPC_STLU
> r1,-INT_FRAME_SIZE(r1)
> plus SAVE_GPR/SAVE_EVR/REST_GPR/REST_EVR?
>
From what I understand INT_FRAME_SIZE is for interrupt entry code. This
is not the case of your code which is a standard function except for
the fact that it clobbers the upper 32 bits of some registers by using
SPE instructions. Therore INT_FRAME_SIZE is overkill. I also believe that
you can save the registers as you suggest, no need to split it into
the high and low part.
By the way, I wonder where the SAVE_EVR/REST_EVR macros are used. I only
see the definitions, no use in a 3.18 source tree.
Gabriel
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