Re: [patch 1/2] QEMU: use SIGARLM for alarm timers, enable SIGIO on qemu_set_fd_handler2()

[Anthony Liguori <aliguori@us.ibm.com>]

With the IO thread, shouldn't we be striving to perform the select()s 
within the IO thread itself to completely avoid the need to use SIGIO at 
all?

Regards,

Anthony Liguori

Marcelo Tosatti wrote:
> From: Anders Melchiorsen <mail@flac.kalibalik.dk>
>
> 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.
>
> [1] The slirp file descriptors are not included yet.
>
>
> Index: kvm-userspace.io/qemu/vl.c
> ===================================================================
> --- kvm-userspace.io.orig/qemu/vl.c
> +++ kvm-userspace.io/qemu/vl.c
> @@ -1177,6 +1177,25 @@ static int timer_load(QEMUFile *f, void 
>      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)
> @@ -1270,7 +1289,20 @@ 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;
>
> @@ -1279,8 +1311,9 @@ 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());
>  }
>
> @@ -1317,7 +1350,7 @@ static int hpet_start_timer(struct qemu_
>      if (r < 0)
>          goto fail;
>
> -    enable_sigio_timer(fd);
> +    enable_sigalrm(fd);
>      t->priv = (void *)(long)fd;
>
>      return 0;
> @@ -1355,7 +1388,7 @@ static int rtc_start_timer(struct qemu_a
>          return -1;
>      }
>
> -    enable_sigio_timer(rtc_fd);
> +    enable_sigalrm(rtc_fd);
>
>      t->priv = (void *)(long)rtc_fd;
>
> @@ -4029,7 +4062,6 @@ static TAPState *net_tap_fd_init(VLANSta
>          return NULL;
>      s->fd = fd;
>      s->no_poll = 0;
> -    enable_sigio_timer(fd);
>      s->vc = qemu_new_vlan_client(vlan, tap_receive, NULL, s);
>      qemu_set_fd_handler2(s->fd, tap_read_poll, tap_send, NULL, s);
>      snprintf(s->vc->info_str, sizeof(s->vc->info_str), "tap: fd=%d", fd);
> @@ -5661,6 +5693,10 @@ 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;
>
>   


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