On Fri, Sep 02, 2016 at 12:02:55PM +0530, Nikunj A Dadhania wrote: > Signed-off-by: Nikunj A Dadhania <nik...@linux.vnet.ibm.com>
This really needs a comment indicating that this implementation isn't
strictly correct (although probably good enough in practice).
Specifically a racing store which happens to store the same value
which was already in memory should clobber the reservation, but won't
with this implementation.
I had a long discussion at KVM Forum with Emilio Costa about this, in
which I discovered just how hard it is to strictly implement
store-conditional semantics in terms of anything else. So, this is
probably a reasonable substitute, but we should note the fact that
it's not 100%.
> ---
> target-ppc/translate.c | 24 +++++++++++++++++++++---
> 1 file changed, 21 insertions(+), 3 deletions(-)
>
> diff --git a/target-ppc/translate.c b/target-ppc/translate.c
> index 4a882b3..447c13e 100644
> --- a/target-ppc/translate.c
> +++ b/target-ppc/translate.c
> @@ -72,6 +72,7 @@ static TCGv cpu_cfar;
> #endif
> static TCGv cpu_xer, cpu_so, cpu_ov, cpu_ca;
> static TCGv cpu_reserve;
> +static TCGv cpu_reserve_val;
> static TCGv cpu_fpscr;
> static TCGv_i32 cpu_access_type;
>
> @@ -176,6 +177,9 @@ void ppc_translate_init(void)
> cpu_reserve = tcg_global_mem_new(cpu_env,
> offsetof(CPUPPCState, reserve_addr),
> "reserve_addr");
> + cpu_reserve_val = tcg_global_mem_new(cpu_env,
> + offsetof(CPUPPCState, reserve_val),
> + "reserve_val");
>
> cpu_fpscr = tcg_global_mem_new(cpu_env,
> offsetof(CPUPPCState, fpscr), "fpscr");
> @@ -3086,7 +3090,7 @@ static void gen_##name(DisasContext *ctx)
> \
> } \
> tcg_gen_qemu_ld_tl(gpr, t0, ctx->mem_idx, memop); \
> tcg_gen_mov_tl(cpu_reserve, t0); \
> - tcg_gen_st_tl(gpr, cpu_env, offsetof(CPUPPCState, reserve_val)); \
> + tcg_gen_mov_tl(cpu_reserve_val, gpr); \
> tcg_temp_free(t0); \
> }
>
> @@ -3112,14 +3116,28 @@ static void gen_conditional_store(DisasContext *ctx,
> TCGv EA,
> int reg, int memop)
> {
> TCGLabel *l1;
> + TCGv_i32 tmp = tcg_temp_local_new_i32();
> + TCGv t0;
>
> + tcg_gen_movi_i32(tmp, 0);
> tcg_gen_trunc_tl_i32(cpu_crf[0], cpu_so);
> l1 = gen_new_label();
> tcg_gen_brcond_tl(TCG_COND_NE, EA, cpu_reserve, l1);
> - tcg_gen_ori_i32(cpu_crf[0], cpu_crf[0], 1 << CRF_EQ);
> - tcg_gen_qemu_st_tl(cpu_gpr[reg], EA, ctx->mem_idx, memop);
> +
> + t0 = tcg_temp_new();
> + tcg_gen_atomic_cmpxchg_tl(t0, EA, cpu_reserve_val, cpu_gpr[reg],
> + ctx->mem_idx, DEF_MEMOP(memop));
> + tcg_gen_setcond_tl(TCG_COND_EQ, t0, t0, cpu_reserve_val);
> + tcg_gen_trunc_tl_i32(tmp, t0);
> +
> gen_set_label(l1);
> + tcg_gen_shli_i32(tmp, tmp, CRF_EQ);
> + tcg_gen_or_i32(cpu_crf[0], cpu_crf[0], tmp);
> tcg_gen_movi_tl(cpu_reserve, -1);
> + tcg_gen_movi_tl(cpu_reserve_val, 0);
> +
> + tcg_temp_free(t0);
> + tcg_temp_free_i32(tmp);
> }
> #endif
>
--
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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