Re: [Qemu-devel] [Qemu-ppc] [PATCH 16/77] ppc: Get out of emulation on SMT "OR" ops

[David Gibson]

On Wed, Nov 11, 2015 at 11:27:29AM +1100, Benjamin Herrenschmidt wrote:
> Otherwise tight loops at smt_low for example, which OPAL does,
> eat so much CPU that we can't boot a kernel anymore. With that,
> I can boot 8 CPUs just fine with powernv.
> 
> Signed-off-by: Benjamin Herrenschmidt <address@hidden>

Reviewed-by: David Gibson <address@hidden>

> ---
>  target-ppc/translate.c | 19 +++++++++++++++++--
>  1 file changed, 17 insertions(+), 2 deletions(-)
> 
> diff --git a/target-ppc/translate.c b/target-ppc/translate.c
> index 3974cd2..e8bbd59 100644
> --- a/target-ppc/translate.c
> +++ b/target-ppc/translate.c
> @@ -1396,6 +1396,19 @@ GEN_LOGICAL2(nand, tcg_gen_nand_tl, 0x0E, PPC_INTEGER);
>  /* nor & nor. */
>  GEN_LOGICAL2(nor, tcg_gen_nor_tl, 0x03, PPC_INTEGER);
>  
> +#if defined(TARGET_PPC64)
> +static void gen_pause(DisasContext *ctx)
> +{
> +    TCGv_i32 t0 = tcg_const_i32(0);
> +    tcg_gen_st_i32(t0, cpu_env,
> +                   -offsetof(PowerPCCPU, env) + offsetof(CPUState, halted));
> +    tcg_temp_free_i32(t0);
> +
> +    /* Stop translation, this gives other CPUs a chance to run */
> +    gen_exception_err(ctx, EXCP_HLT, 1);
> +}
> +#endif /* defined(TARGET_PPC64) */
> +
>  /* or & or. */
>  static void gen_or(DisasContext *ctx)
>  {
> @@ -1468,6 +1481,10 @@ 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
> +             */
> +            gen_pause(ctx);
>          }
>  #endif
>      }
> @@ -1493,8 +1510,6 @@ static void gen_ori(DisasContext *ctx)
>      target_ulong uimm = UIMM(ctx->opcode);
>  
>      if (rS(ctx->opcode) == rA(ctx->opcode) && uimm == 0) {
> -        /* NOP */
> -        /* XXX: should handle special NOPs for POWER series */
>          return;
>      }
>      tcg_gen_ori_tl(cpu_gpr[rA(ctx->opcode)], cpu_gpr[rS(ctx->opcode)], uimm);

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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