[RFC][PATCH] powerpc/64s: stop using r14 register

[Naveen N. Rao]

On 2017/05/22 12:00AM, Nicholas Piggin wrote:
> I'd like to take over the r14 register for use as a per-cpu kernel
> register similar to the way r13 is used for the paca.
> 
> r14 being the last non-volatile register gcc allocates, appears with
> about 0.5% the frequency as r31 in (static) instructions. I haven't
> counted dynamically how many extra spills and fills that removing it
> causes, but I should. My guess is the memory ops saved by using
> it as a per-cpu variable will significantly outweigh the cost of
> losing it as a general use register.
> 
> This part of the patch is pretty mechanical. A couple of places (prom)
> still have to use it, and I haven't quite understood the KVM code yet.
> 
> Question is whether this approach seems okay, and whether we should do
> the same for 64e.
> 
> Thanks,
> Nick
> 
> ---

[snip]

> diff --git a/arch/powerpc/net/bpf_jit32.h 
> b/arch/powerpc/net/bpf_jit32.h
> index a8cd7e289ecd..52a30db033c1 100644
> --- a/arch/powerpc/net/bpf_jit32.h
> +++ b/arch/powerpc/net/bpf_jit32.h
> @@ -44,9 +44,11 @@
>   * A register	r4
>   * X register	r5
>   * addr param	r6
> - * r7-r10	scratch
> - * skb->data	r14
> - * skb headlen	r15	(skb->len - skb->data_len)
> + * scratch	r7-r8
> + * skb headlen	r9	(skb->len - skb->data_len)
> + * skb->data	r10
> + * fixed regs	r13-r14
> + * unused	r15
>   * m[0]		r16
>   * m[...]	...
>   * m[15]	r31
> @@ -58,8 +60,8 @@
>  #define r_addr		6
>  #define r_scratch1	7
>  #define r_scratch2	8
> -#define r_D		14
> -#define r_HL		15
> +#define r_HL		9
> +#define r_D		10

You'll also need changes in the JIT code itself, at least in 
bpf_jit_build_prologue() and elsewhere -- some code expects r_D and r_HL 
to be NVRs. It's probably easier to just choose other NVRs here...

>  #define r_M		16
> 
>  #ifndef __ASSEMBLY__
> diff --git a/arch/powerpc/net/bpf_jit_asm.S b/arch/powerpc/net/bpf_jit_asm.S
> index 3dd9c43d40c9..5b06152052f6 100644
> --- a/arch/powerpc/net/bpf_jit_asm.S
> +++ b/arch/powerpc/net/bpf_jit_asm.S
> @@ -19,8 +19,8 @@
>   * r3		skb
>   * r4,r5	A,X
>   * r6		*** address parameter to helper ***
> - * r7-r10	scratch
> - * r14		skb->data
> + * r7-r9	scratch
> + * r10		skb->data
>   * r15		skb headlen
>   * r16-31	M[]

This doesn't match the updates to bpf_jit32.h.

- Naveen