Re: [Qemu-devel] [PATCH] qemu: generate signals on tap I/O

[Aurelien Jarno <aurelien@aurel32.net>]

Anders Melchiorsen a écrit :
> Aurelien Jarno <aurelien@aurel32.net> writes:
> 
>> The patch below from KVM improves network transfers in a huge way. 
>> wget in a MIPS target now gives me a transfer speed of up to 120
>> Mbits/s with an e1000 emulated card.
> 
> Without I/O signals, qemu is relying on periodic timer events to poll
> the I/O. That seems wrong, even though it works reasonably well
> because timers are so frequent. In KVM, timers are less frequent, and
> it does not work quite as well.
> 
> Here is a quick try at a more elaborate patch.
> 
> It attaches a signal to all[1] file descriptors that will be used in
> select(). Also, it uses a dedicated SIGIO handler rather than
> piggybacking on the alarm handler, so alarm I/O is changed to use
> SIGALRM.
> 
> I copied the handler function from the alarm case, quite frankly I do
> not quite understand what is going on. Also, I left _WIN32 out, since
> I have no idea how signals work there.
> 

I have just given a tried to your patch, which looks indeed more
complete. I have made some performance measurement
(qemu-system-mips64el, 2.6.24 5kc kernel), I am getting the same
performances as with my patches on the network side, and a very small
improvement (< 3%) on the hard disk performance, but it can also be
noise (though the measurements looked quite stable).

Anybody to commit this patch?

> 
> diff --git a/vl.c b/vl.c
> index c87e8bc..ff8ceef 100644
> --- a/vl.c
> +++ b/vl.c
> @@ -1148,6 +1148,25 @@ static int timer_load(QEMUFile *f, void *opaque, int version_id)
>      return 0;
>  }
>  
> +#ifndef _WIN32
> +static void host_io_handler(int host_signum)
> +{
> +    CPUState *env = next_cpu;
> +
> +    if (env) {
> +        /* stop the currently executing cpu because io occured */
> +        cpu_interrupt(env, CPU_INTERRUPT_EXIT);
> +#ifdef USE_KQEMU
> +        if (env->kqemu_enabled) {
> +            kqemu_cpu_interrupt(env);
> +        }
> +#endif
> +    }
> +
> +    event_pending = 1;
> +}
> +#endif
> +
>  #ifdef _WIN32
>  void CALLBACK host_alarm_handler(UINT uTimerID, UINT uMsg,
>                                   DWORD_PTR dwUser, DWORD_PTR dw1, DWORD_PTR dw2)
> @@ -1240,7 +1259,20 @@ static uint64_t qemu_next_deadline(void)
>  
>  #define RTC_FREQ 1024
>  
> -static void enable_sigio_timer(int fd)
> +static void enable_sigio(int fd)
> +{
> +    struct sigaction act;
> +
> +    sigfillset(&act.sa_mask);
> +    act.sa_flags = 0;
> +    act.sa_handler = host_io_handler;
> +
> +    sigaction(SIGIO, &act, NULL);
> +    fcntl(fd, F_SETFL, O_ASYNC);
> +    fcntl(fd, F_SETOWN, getpid());
> +}
> +
> +static void enable_sigalrm(int fd)
>  {
>      struct sigaction act;
>  
> @@ -1249,8 +1281,9 @@ static void enable_sigio_timer(int fd)
>      act.sa_flags = 0;
>      act.sa_handler = host_alarm_handler;
>  
> -    sigaction(SIGIO, &act, NULL);
> +    sigaction(SIGALRM, &act, NULL);
>      fcntl(fd, F_SETFL, O_ASYNC);
> +    fcntl(fd, F_SETSIG, SIGALRM);
>      fcntl(fd, F_SETOWN, getpid());
>  }
>  
> @@ -1287,7 +1320,7 @@ static int hpet_start_timer(struct qemu_alarm_timer *t)
>      if (r < 0)
>          goto fail;
>  
> -    enable_sigio_timer(fd);
> +    enable_sigalrm(fd);
>      t->priv = (void *)(long)fd;
>  
>      return 0;
> @@ -1325,7 +1358,7 @@ static int rtc_start_timer(struct qemu_alarm_timer *t)
>          return -1;
>      }
>  
> -    enable_sigio_timer(rtc_fd);
> +    enable_sigalrm(rtc_fd);
>  
>      t->priv = (void *)(long)rtc_fd;
>  
> @@ -5481,6 +5514,10 @@ int qemu_set_fd_handler2(int fd,
>              return -1;
>          ioh->next = first_io_handler;
>          first_io_handler = ioh;
> +#ifndef _WIN32
> +        enable_sigio(fd);
> +#endif
> +
>      found:
>          ioh->fd = fd;
>          ioh->fd_read_poll = fd_read_poll;
> 


-- 
  .''`.  Aurelien Jarno	            | GPG: 1024D/F1BCDB73
 : :' :  Debian developer           | Electrical Engineer
 `. `'   aurel32@debian.org         | aurelien@aurel32.net
   `-    people.debian.org/~aurel32 | www.aurel32.net