Re: [PATCH 5/8] powerpc: Restore FPU/VEC/VSX if previously used

[Michael Ellerman <mpe@ellerman.id.au>]

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