Re: [Qemu-devel] [PATCH for-4.2 14/14] icount: clean up cpu_can_io before jumping to the next block

[Paolo Bonzini <pbonzini@redhat.com>]

On 24/07/19 10:44, Pavel Dovgalyuk wrote:
> From: Pavel Dovgalyuk <Pavel.Dovgaluk@ispras.ru>
> 
> Most of IO instructions can be executed only at the end of the block in
> icount mode. Therefore translator can set cpu_can_io flag when translating
> the last instruction.
> But when the blocks are chained, then this flag is not reset and may
> remain set at the beginning of the next block.
> This patch resets the flag before "chaining" the translation blocks.
> 
> Signed-off-by: Pavel Dovgalyuk <Pavel.Dovgaluk@ispras.ru>
> ---
>  accel/tcg/tcg-runtime.c |    2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/accel/tcg/tcg-runtime.c b/accel/tcg/tcg-runtime.c
> index 8a1e408e31..fe6b83d0fc 100644
> --- a/accel/tcg/tcg-runtime.c
> +++ b/accel/tcg/tcg-runtime.c
> @@ -151,6 +151,8 @@ void *HELPER(lookup_tb_ptr)(CPUArchState *env)
>      target_ulong cs_base, pc;
>      uint32_t flags;
>  
> +    /* We are going to jump to the next block. can_do_io should be reset */
> +    cpu->can_do_io = !use_icount;
>      tb = tb_lookup__cpu_state(cpu, &pc, &cs_base, &flags, curr_cflags());
>      if (tb == NULL) {
>          return tcg_ctx->code_gen_epilogue;
> 

This only fixes indirect jumps though.

I think you do not need this patch if you remove the assignment in
cpu_tb_exec, and compile a "move 0 to cpu->can_do_io" in gen_tb_start
instead.

Paolo