Re: [Qemu-devel] [Qemu-ppc] [PATCH] target-ppc: gen_pause for instructions: yield, mdoio, mdoom, miso

[Benjamin Herrenschmidt <benh@au1.ibm.com>]

On Fri, 2016-06-24 at 13:18 -0700, Aaron Larson wrote:
> Call gen_pause for all "or rx,rx,rx" encodings other nop.  This
> provides a reasonable implementation for yield, and a better
> approximation for mdoio, mdoom, and miso.  The choice to pause for
> all
> encodings !=0 leverages the PowerISA admonition that the reserved
> encodings might change program priority, providing a slight "future
> proofing".
> 
> Signed-off-by: Aaron Larson <alarson@ddci.com>

Acked-by: Benjamin Herrenschmidt <benh@kerne.crashing.org>

> ---
>  target-ppc/translate.c | 15 ++++++++-------
>  1 file changed, 8 insertions(+), 7 deletions(-)
> 
> diff --git a/target-ppc/translate.c b/target-ppc/translate.c
> index 2f1c591..c4559b6 100644
> --- a/target-ppc/translate.c
> +++ b/target-ppc/translate.c
> @@ -1471,7 +1471,7 @@ static void gen_or(DisasContext *ctx)
>      } else if (unlikely(Rc(ctx->opcode) != 0)) {
>          gen_set_Rc0(ctx, cpu_gpr[rs]);
>  #if defined(TARGET_PPC64)
> -    } else {
> +    } else if (rs != 0) { /* 0 is nop */
>          int prio = 0;
>  
>          switch (rs) {
> @@ -1514,7 +1514,6 @@ static void gen_or(DisasContext *ctx)
>              break;
>  #endif
>          default:
> -            /* nop */
>              break;
>          }
>          if (prio) {
> @@ -1524,13 +1523,15 @@ static void gen_or(DisasContext *ctx)
>              tcg_gen_ori_tl(t0, t0, ((uint64_t)prio) << 50);
>              gen_store_spr(SPR_PPR, t0);
>              tcg_temp_free(t0);
> -            /* Pause us out of TCG otherwise spin loops with smt_low
> -             * eat too much CPU and the kernel hangs
> -             */
> +        }
>  #if !defined(CONFIG_USER_ONLY)
> -            gen_pause(ctx);
> +        /* Pause out of TCG otherwise spin loops with smt_low eat
> too much
> +         * CPU and the kernel hangs.  This applies to all encodings
> other
> +         * than no-op, e.g., miso(rs=26), yield(27), mdoio(29),
> mdoom(30),
> +         * and all currently undefined.
> +         */
> +        gen_pause(ctx);
>  #endif
> -        }
>  #endif
>      }
>  }