On Fri, 20 Nov 2015 22:01:04 +1100
Michael Ellerman <m...@ellerman.id.au> wrote:
> On Wed, 2015-11-18 at 14:26 +1100, Cyril Bur wrote:
> > diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
> > index c8b4225..46e9869 100644
> > --- a/arch/powerpc/kernel/entry_64.S
> > +++ b/arch/powerpc/kernel/entry_64.S
> > @@ -210,7 +210,54 @@ system_call: /* label this so stack
> > traces look sane */
> > li r11,-MAX_ERRNO
> > andi.
> > r0,r9,(_TIF_SYSCALL_DOTRACE|_TIF_SINGLESTEP|_TIF_USER_WORK_MASK|_TIF_PERSYSCALL_MASK)
> > bne- syscall_exit_work
> > - cmpld r3,r11
> > +
> > + /*
> > + * This is an assembly version of checks performed in restore_math()
> > + * to avoid calling C unless absolutely necessary.
> > + * Note: In order to simplify the assembly, if the FP or VEC registers
> > + * are hot (and therefore restore_math() isn't called) the
> > + * LOAD_{FP,VEC} thread counter doesn't get incremented.
> > + * This is likely the best thing to do anyway because hot regs indicate
> > + * that the workload is doing a lot of syscalls that can be handled
> > + * quickly and without the need to touch FP or VEC regs (by the kernel).
> > + * a) If this workload is long running then this is exactly what the
> > + * kernel should be doing.
> > + * b) If this workload isn't long running then we'll soon fall back to
> > + * calling into C and the counter will be incremented regularly again
> > + * anyway.
> > + */
> > + ld r9,PACACURRENT(r13)
> > + andi. r0,r8,MSR_FP
> > + addi r9,r9,THREAD
> > + lbz r5,THREAD_LOAD_FP(r9)
> > + /*
> > + * Goto 2 if !r0 && r5
> > + * The cmpb works because r5 can only have bits set in the lowest byte
> > + * and r0 may or may not have bit 13 set (different byte) but will have
> > + * a zero low byte therefore the low bytes must differ if r5 == true
> > + * and the bit 13 byte must be the same if !r0
> > + */
> > + cmpb r7,r0,r5
>
> cmpb is new since Power6, which means it doesn't exist on Cell -> Program
> Check :)
>
Oops, sorry.
> I'm testing a patch using crandc, but I don't like it.
>
> I'm not a big fan of the logic here, it's unpleasantly complicated. Did you
> benchmark going to C to do the checks? Or I wonder if we could just check
> THREAD_LOAD_FP || THREAD_LOAD_VEC and if either is set we go to
> restore_math().
>
I didn't benchmark going to C mostly because you wanted to avoid calling C
unless necessary in that path. Based off the results I got benchmarking the
this series I expect calling C will also be in the noise of removing the
exception.
> Or on the other hand we check !MSR_FP && !MSR_VEC and if so we go to
> restore_math()?
>
That seems like the best check to leave in the assembly if you want to avoid
complicated assembly in there.
> > + cmpldi r7,0xff0
> > +#ifdef CONFIG_ALTIVEC
> > + beq 2f
> > +
> > + lbz r9,THREAD_LOAD_VEC(r9)
> > + andis. r0,r8,MSR_VEC@h
> > + /* Skip (goto 3) if r0 || !r9 */
> > + bne 3f
> > + cmpldi r9,0
> > + beq 3f
> > +#else
> > + bne 3f
> > +#endif
> > +2: addi r3,r1,STACK_FRAME_OVERHEAD
> > + bl restore_math
> > + ld r8,_MSR(r1)
> > + ld r3,RESULT(r1)
> > + li r11,-MAX_ERRNO
> > +
> > +3: cmpld r3,r11
> > ld r5,_CCR(r1)
> > bge- syscall_error
> > .Lsyscall_error_cont:
>
>
> cheers
>
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