Re: [Qemu-devel] [PATCH 19/22] tcg: introduce tcg_context_clone

[Alex Bennée]

Emilio G. Cota <address@hidden> writes:

> Before we make TCGContext thread-local.
>
> Signed-off-by: Emilio G. Cota <address@hidden>
> ---
>  tcg/tcg.h |  1 +
>  tcg/tcg.c | 14 ++++++++++++++
>  2 files changed, 15 insertions(+)
>
> diff --git a/tcg/tcg.h b/tcg/tcg.h
> index 2a64ee2..be5f3fd 100644
> --- a/tcg/tcg.h
> +++ b/tcg/tcg.h
> @@ -778,6 +778,7 @@ static inline void *tcg_malloc(int size)
>  }
>
>  void tcg_context_init(TCGContext *s);
> +void tcg_context_clone(TCGContext *s);
>  void tcg_prologue_init(TCGContext *s);
>  void tcg_register_thread(void);
>  void tcg_func_start(TCGContext *s);
> diff --git a/tcg/tcg.c b/tcg/tcg.c
> index 2f003a0..8febf53 100644
> --- a/tcg/tcg.c
> +++ b/tcg/tcg.c
> @@ -117,6 +117,7 @@ static bool tcg_out_tb_finalize(TCGContext *s);
>
>  #define TCG_HIGHWATER 1024
>
> +static const TCGContext *tcg_init_ctx;
>  static QemuMutex tcg_lock;
>
>  /*
> @@ -353,6 +354,7 @@ void tcg_context_init(TCGContext *s)
>      TCGArgConstraint *args_ct;
>      int *sorted_args;
>
> +    tcg_init_ctx = s;
>      memset(s, 0, sizeof(*s));
>      s->nb_globals = 0;
>
> @@ -409,6 +411,18 @@ void tcg_context_init(TCGContext *s)
>  }
>
>  /*
> + * Clone the initial TCGContext. Used by TCG threads to copy the TCGContext
> + * set up by their parent thread via tcg_context_init().
> + */
> +void tcg_context_clone(TCGContext *s)
> +{
> +    if (unlikely(tcg_init_ctx == NULL || tcg_init_ctx == s)) {
> +        tcg_abort();
> +    }
> +    memcpy(s, tcg_init_ctx, sizeof(*s));
> +}
> +

Why a copy approach as opposed to a plain tcg_context_new() with the
appropriate init cleanup. Is is it just because of the extra shared
stuff in:

#if defined(CONFIG_SOFTMMU)
    /* There's no guest base to take into account, so go ahead and
       initialize the prologue now.  */
    tcg_prologue_init(&tcg_ctx);
    code_gen_set_region_size(&tcg_ctx);
#endif

?

> +/*
>   * Allocate TBs right before their corresponding translated code, making
>   * sure that TBs and code are on different cache lines.
>   */


--
Alex Bennée