Re: [Qemu-devel] [PATCH] target-sh4: add atomic tas

[Paolo Bonzini]

On 03/11/2016 14:12, Laurent Vivier wrote:
> Implement real atomic tas.
> 
> As tas is:
> 
>     When (Rn) = 0, 1 -> T
>     Otherwise, 0 -> T
>     In both cases, 1 -> MSB of (Rn)
> 
> It can't be implemented using cmpxchg().

Why not?

This is essentially

    movi                temp0, 0x80
    atomic_fetch_or_i32 temp1, Rn, temp0, ctx->memidx, MO_UB
    setcondi_i32        cpu_sr_t, TCG_COND_EQ, temp1, 0

But the patch looks good anyway.

Paolo

> So we use an helper and "parallel_cpus"+cpu_loop_exit_atomic().
> 
> Tested with image from:
> http://wiki.qemu.org/download/sh-test-0.2.tar.bz2
> 
> This image contains a "tas_test" that runs without
> error with this change.
> 
> Signed-off-by: Laurent Vivier <address@hidden>
> ---
>  target-sh4/helper.h    |  1 +
>  target-sh4/op_helper.c | 24 ++++++++++++++++++++++++
>  target-sh4/translate.c | 13 +------------
>  3 files changed, 26 insertions(+), 12 deletions(-)
> 
> diff --git a/target-sh4/helper.h b/target-sh4/helper.h
> index dce859c..4bfa98c 100644
> --- a/target-sh4/helper.h
> +++ b/target-sh4/helper.h
> @@ -6,6 +6,7 @@ DEF_HELPER_1(raise_slot_fpu_disable, noreturn, env)
>  DEF_HELPER_1(debug, noreturn, env)
>  DEF_HELPER_1(sleep, noreturn, env)
>  DEF_HELPER_2(trapa, noreturn, env, i32)
> +DEF_HELPER_2(tas, void, env, i32)
>  
>  DEF_HELPER_3(movcal, void, env, i32, i32)
>  DEF_HELPER_1(discard_movcal_backup, void, env)
> diff --git a/target-sh4/op_helper.c b/target-sh4/op_helper.c
> index 684d3f3..17afa92 100644
> --- a/target-sh4/op_helper.c
> +++ b/target-sh4/op_helper.c
> @@ -105,6 +105,30 @@ void helper_trapa(CPUSH4State *env, uint32_t tra)
>      raise_exception(env, 0x160, 0);
>  }
>  
> +void helper_tas(CPUSH4State *env, uint32_t addr)
> +{
> +    uint8_t l;
> +    uintptr_t ra = GETPC();
> +
> +    if (parallel_cpus) {
> +        /* Tell the main loop we need to serialize this insn.  */
> +        cpu_loop_exit_atomic(ENV_GET_CPU(env), ra);
> +    } else {
> +        /* We're executing in a serial context -- no need to be atomic.  */
> +#ifdef CONFIG_USER_ONLY
> +        uint8_t *haddr = g2h(addr);
> +        l = ldub_p(haddr);
> +        stb_p(haddr, l | 0x80);
> +#else
> +        int mmu_idx = cpu_mmu_index(env, 0);
> +        TCGMemOpIdx oi = make_memop_idx(MO_UB, mmu_idx);
> +        l = helper_ret_ldub_mmu(env, addr, oi, ra);
> +        helper_ret_stb_mmu(env, addr,  l | 0x80, oi, ra);
> +#endif
> +        env->sr_t = l == 0;
> +    }
> +}
> +
>  void helper_movcal(CPUSH4State *env, uint32_t address, uint32_t value)
>  {
>      if (cpu_sh4_is_cached (env, address))
> diff --git a/target-sh4/translate.c b/target-sh4/translate.c
> index c89a147..44238fc 100644
> --- a/target-sh4/translate.c
> +++ b/target-sh4/translate.c
> @@ -1640,18 +1640,7 @@ static void _decode_opc(DisasContext * ctx)
>       tcg_gen_shri_i32(REG(B11_8), REG(B11_8), 16);
>       return;
>      case 0x401b:             /* tas.b @Rn */
> -     {
> -         TCGv addr, val;
> -         addr = tcg_temp_local_new();
> -         tcg_gen_mov_i32(addr, REG(B11_8));
> -         val = tcg_temp_local_new();
> -            tcg_gen_qemu_ld_i32(val, addr, ctx->memidx, MO_UB);
> -            tcg_gen_setcondi_i32(TCG_COND_EQ, cpu_sr_t, val, 0);
> -         tcg_gen_ori_i32(val, val, 0x80);
> -            tcg_gen_qemu_st_i32(val, addr, ctx->memidx, MO_UB);
> -         tcg_temp_free(val);
> -         tcg_temp_free(addr);
> -     }
> +        gen_helper_tas(cpu_env, REG(B11_8));
>       return;
>      case 0xf00d: /* fsts FPUL,FRn - FPSCR: Nothing */
>       CHECK_FPU_ENABLED
>