Re: [Qemu-devel] [PATCH v7 04/14] icount: exit cpu loop on expire

[Paolo Bonzini]

On 24/01/2017 08:17, Pavel Dovgalyuk wrote:
> This patch adds check to break cpu loop when icount expires without
> setting the TB_EXIT_ICOUNT_EXPIRED flag. It happens when there is no
> available translated blocks and all instructions were executed.
> In icount replay mode unnecessary tb_find will be called (which may
> cause an exception) and execution will be non-deterministic.
> 
> Signed-off-by: Pavel Dovgalyuk <address@hidden>
> ---
>  cpu-exec.c |   15 ++++++++++++---
>  1 file changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/cpu-exec.c b/cpu-exec.c
> index 79a2167..e603da4 100644
> --- a/cpu-exec.c
> +++ b/cpu-exec.c
> @@ -582,9 +582,6 @@ static inline void cpu_loop_exec_tb(CPUState *cpu, 
> TranslationBlock *tb,
>                  cpu_exec_nocache(cpu, insns_left, *last_tb, false);
>                  align_clocks(sc, cpu);
>              }
> -            cpu->exception_index = EXCP_INTERRUPT;
> -            *last_tb = NULL;
> -            cpu_loop_exit(cpu);
>          }
>          break;
>  #endif
> @@ -638,6 +635,18 @@ int cpu_exec(CPUState *cpu)
>  
>              for(;;) {
>                  cpu_handle_interrupt(cpu, &last_tb);
> +                /* icount has expired, we need to break the execution loop.
> +                   This check is needed before tb_find to make execution
> +                   deterministic - tb_find may cause an exception
> +                   while translating the code from non-mapped page. */
> +                if (use_icount && ((cpu->icount_extra == 0
> +                                    && cpu->icount_decr.u16.low == 0)
> +                                || (int32_t)cpu->icount_decr.u32 < 0)) {
> +                    if (cpu->exception_index == -1) {
> +                        cpu->exception_index = EXCP_INTERRUPT;
> +                    }
> +                    cpu_loop_exit(cpu);
> +                }

Can this can be placed in cpu_handle_interrupt itself?  Also, can the
cpu->exception_index != -1 case happen?

We really should add assertions on cpu->exception_index, maybe after
cpu_handle_exception and cc->cpu_exec_interrupt returns true.  Without
some idea of the invariants, I'm not competent enough to review this patch.

Paolo

>                  tb = tb_find(cpu, last_tb, tb_exit);
>                  cpu_loop_exec_tb(cpu, tb, &last_tb, &tb_exit, &sc);
>                  /* Try to align the host and virtual clocks
>