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

[Paolo Bonzini]

On 26/04/2017 12:29, Alex Bennée wrote:
> 
> Emilio G. Cota <address@hidden> writes:
> 
>> 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;
>> +    }
>> +    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)) {
> 
> Should we also not be checking the TB hasn't been invalidated: tb->invalid?

It's not needed because this lookup is (if I understand it right) once
only and is not reused later.  This is why tb_find doesn't check
tb->invalid, but uses it to avoid adding the TB to the chain.

Good:

        tb_find                 tb_phys_invalidate
                                  tb_lock
                                  tb->invalid = true
          lookup cache
          cache hit
                                  tb_unlock
          tb_lock
          tb->invalid?
            yes, skip tb_add_jump
          tb_unlock
          execute tb once

Bad (doesn't happen):

        tb_find                 tb_phys_invalidate
                                  tb_lock
                                  tb->invalid = true
          lookup cache
          cache hit
                                  tb_unlock
          tb_lock
          tb_add_jump
          tb_unlock
          execute tb many times

Paolo

>> +        return tb->tc_ptr;
>> +    }
>> + out_epilogue:
>> +    return tcg_ctx.code_gen_epilogue;
>> +}
>> +
>>  void HELPER(exit_atomic)(CPUArchState *env)
>>  {
>>      cpu_loop_exit_atomic(ENV_GET_CPU(env), GETPC());
>> diff --git a/tcg/tcg-runtime.h b/tcg/tcg-runtime.h
>> index 114ea6f..c41d38a 100644
>> --- a/tcg/tcg-runtime.h
>> +++ b/tcg/tcg-runtime.h
>> @@ -24,6 +24,8 @@ DEF_HELPER_FLAGS_1(clrsb_i64, TCG_CALL_NO_RWG_SE, i64, i64)
>>  DEF_HELPER_FLAGS_1(ctpop_i32, TCG_CALL_NO_RWG_SE, i32, i32)
>>  DEF_HELPER_FLAGS_1(ctpop_i64, TCG_CALL_NO_RWG_SE, i64, i64)
>>
>> +DEF_HELPER_FLAGS_2(lookup_tb_ptr, TCG_CALL_NO_WG_SE, ptr, env, tl)
>> +
>>  DEF_HELPER_FLAGS_1(exit_atomic, TCG_CALL_NO_WG, noreturn, env)
>>
>>  #ifdef CONFIG_SOFTMMU
>> diff --git a/tcg/tcg.h b/tcg/tcg.h
>> index 6c216bb..5ec48d1 100644
>> --- a/tcg/tcg.h
>> +++ b/tcg/tcg.h
>> @@ -699,6 +699,7 @@ struct TCGContext {
>>         extension that allows arithmetic on void*.  */
>>      int code_gen_max_blocks;
>>      void *code_gen_prologue;
>> +    void *code_gen_epilogue;
>>      void *code_gen_buffer;
>>      size_t code_gen_buffer_size;
>>      void *code_gen_ptr;
> 
> 
> --
> Alex Bennée
>