Re: [Qemu-devel] [PATCH v3 01/10] tcg-runtime: add lookup_tb_ptr helper

[Richard Henderson]

On 04/26/2017 08:23 AM, Emilio G. Cota wrote:
> This paves the way for upcoming work.
>
> Reviewed-by: Richard Henderson <address@hidden>
> Signed-off-by: Emilio G. Cota <address@hidden>
> ---
>   tcg-runtime.c     | 21 +++++++++++++++++++++
>   tcg/tcg-runtime.h |  2 ++
>   tcg/tcg.h         |  1 +
>   3 files changed, 24 insertions(+)
>
> diff --git a/tcg-runtime.c b/tcg-runtime.c
> index 4c60c96..90d2d4b 100644
> --- a/tcg-runtime.c
> +++ b/tcg-runtime.c
> @@ -27,6 +27,7 @@
>   #include "exec/helper-proto.h"
>   #include "exec/cpu_ldst.h"
>   #include "exec/exec-all.h"
> +#include "exec/tb-hash.h"
>   
>   /* 32-bit helpers */
>   
> @@ -141,6 +142,26 @@ uint64_t HELPER(ctpop_i64)(uint64_t arg)
>       return ctpop64(arg);
>   }
>   
> +void *HELPER(lookup_tb_ptr)(CPUArchState *env, target_ulong addr)
> +{
> +    CPUState *cpu = ENV_GET_CPU(env);
> +    TranslationBlock *tb;
> +    target_ulong cs_base, pc;
> +    uint32_t flags;
> +
> +    if (unlikely(atomic_read(&cpu->exit_request))) {
> +        goto out_epilogue;
> +    }

Paolo is right.  This will also be checked by the first instructions of the TB 
and there's little point in repeating it here, especially if it is indeed unlikely.

> +    cpu_get_tb_cpu_state(env, &pc, &cs_base, &flags);
> +    tb = atomic_rcu_read(&cpu->tb_jmp_cache[tb_jmp_cache_hash_func(addr)]);
> +    if (likely(tb && tb->pc == addr && tb->cs_base == cs_base &&
> +               tb->flags == flags)) {

This comparison is wrong.  It will incorrectly reject a TB for i386 guest when 
CS_BASE != 0.  You really want

  tb = atomic_rcu_read(&cpu->tb_jmp_cache[tb_jmp_cache_hash_func(addr)]);
  if (tb) {
    cpu_get_tb_cpu_state(env, &pc, &cs_base, &flags);
    if (tb->pc == pc && tb->cs_base == cs_base && tb->flags == flags) {
      return tb->tc_ptr;
    }
  }
  return tcg_ctx.code_gen_epilogue;

where you don't even load the cpu state if there isn't a preliminary hit in the 
cache.  (Note to self: That minor optimization would also apply to tb_find.)

I also wonder, if we've gone this far, if we wouldn't go all the way and also 
check tb_htable_lookup.


r~