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
:)
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().
Or on the other hand we check !MSR_FP && !MSR_VEC and if so we go to
restore_math()?
> + 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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