Re: [PATCH 1/2] tcg: Clear plugin_mem_cbs on TB exit

[Emilio Cota <cota@braap.org>]

As I mentioned in the patch that is being superseded here
I like this approach -- it is simpler and generates less
code.

I'd also like to see the plugin_gen_disable_mem_helpers
function go away, and a mention somewhere that now we are
intentionally not clearing cpu->plugin_mem_cbs until TB exit
(before we weren't doing that either, but that was unintentional
due to a bug).  So, for instance when doing a goto_tb from a
TB with helpers, we leave plugin_mem_cbs set. This is not a
problem in practice because if subsequent TB's use helpers,
they will overwrite the pointer.

Some more comments below.

On Tue, Feb 28, 2023 at 16:47:36 -1000, Richard Henderson wrote:
> Do this in cpu_tb_exec (normal exit) and cpu_loop_exit (exception),
> adjacent to where we reset can_do_io.
> 
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1381
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
>  accel/tcg/cpu-exec-common.c | 4 ++++
>  accel/tcg/cpu-exec.c        | 5 +++--
>  2 files changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/accel/tcg/cpu-exec-common.c b/accel/tcg/cpu-exec-common.c
> index c7bc8c6efa..e136b0843c 100644
> --- a/accel/tcg/cpu-exec-common.c
> +++ b/accel/tcg/cpu-exec-common.c
> @@ -65,6 +65,10 @@ void cpu_loop_exit(CPUState *cpu)
>  {
>      /* Undo the setting in cpu_tb_exec.  */
>      cpu->can_do_io = 1;
> +#ifdef CONFIG_PLUGIN
> +    /* Undo any setting in generated code. */
> +    cpu->plugin_mem_cbs = NULL;
> +#endif
>      siglongjmp(cpu->jmp_env, 1);
>  }
>  
> diff --git a/accel/tcg/cpu-exec.c b/accel/tcg/cpu-exec.c
> index 56aaf58b9d..2831fcafee 100644
> --- a/accel/tcg/cpu-exec.c
> +++ b/accel/tcg/cpu-exec.c
> @@ -459,6 +459,9 @@ cpu_tb_exec(CPUState *cpu, TranslationBlock *itb, int *tb_exit)
>      qemu_thread_jit_execute();
>      ret = tcg_qemu_tb_exec(env, tb_ptr);
>      cpu->can_do_io = 1;
> +#ifdef CONFIG_PLUGIN
> +    cpu->plugin_mem_cbs = NULL;
> +#endif

We should use qemu_plugin_disable_mem_helpers, which avoids the
ifdef.

Also note that there are existing calls to that function that
should now go away because they happen after the clearings here.

Thanks,

		Emilio