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

[Anders Melchiorsen]

Aurelien Jarno <address@hidden> 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.


[1] The slirp file descriptors are not included yet.


Anders.




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;