Re: [Qemu-devel] [PATCH v2 11/12] cpus: call cpu_update_icount on read

[Paolo Bonzini]

On 05/04/2017 15:25, Alex Bennée wrote:
> This ensures each time the vCPU thread reads the icount we update the
> master timer_state.qemu_icount field. This way as long as updates are
> in BQL protected sections (which they should be) the main-loop can
> never come to update the log and find time has gone backwards.
> 
> Signed-off-by: Alex Bennée <address@hidden>
> ---
>  cpus.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/cpus.c b/cpus.c
> index ff75af449a..63de033cc8 100644
> --- a/cpus.c
> +++ b/cpus.c
> @@ -246,19 +246,17 @@ void cpu_update_icount(CPUState *cpu)
>  
>  int64_t cpu_get_icount_raw(void)
>  {
> -    int64_t icount;
>      CPUState *cpu = current_cpu;
>  
> -    icount = atomic_read(&timers_state.qemu_icount);
>      if (cpu && cpu->running) {
>          if (!cpu->can_do_io) {
>              fprintf(stderr, "Bad icount read\n");
>              exit(1);
>          }
>          /* Take into account what has run */
> -        icount += cpu_get_icount_executed(cpu);
> +        cpu_update_icount(cpu);
>      }
> -    return icount;
> +    return atomic_read(&timers_state.qemu_icount);
>  }
>  
>  /* Return the virtual CPU time, based on the instruction counter.  */
> 

Maybe the update should be done in gen_io_start instead.  There
shouldn't be any interference between vCPU and I/O threads except
between gen_io_start and gen_io_end.

Paolo