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authorFabien Chouteau <chouteau@adacore.com>2013-01-08 16:30:56 +0100
committerAnthony Liguori <aliguori@us.ibm.com>2013-01-09 11:03:05 -0600
commit5e3bc735d93dd23f074b5116fd11e1ad8cd4962f (patch)
tree384a280b0892ee81c14d00a106627337913748b2 /main-loop.c
parent7cd5da7eef152a533c5774effd2e7bbfa5976c86 (diff)
downloadqemu-arm-5e3bc735d93dd23f074b5116fd11e1ad8cd4962f.tar.gz
Check return values from g_poll and select
The current implementation of os_host_main_loop_wait() on Windows, returns 1 only when a g_poll() event occurs because the return value of select() is overridden. This is wrong as we may skip a socket event, as shown in this example: 1. select() returns 0 2. g_poll() returns 1 (socket event occurs) 3. os_host_main_loop_wait() returns 1 4. qemu_iohandler_poll() sees no socket event because select() has return before the event occurs 5. select() returns 1 6. g_poll() returns 0 (g_poll overrides select's return value) 7. os_host_main_loop_wait() returns 0 8. qemu_iohandler_poll() doesn't check for socket events because the return value of os_host_main_loop_wait() is zero. 9. goto 5 This patch use one variable for each of these return values, so we don't miss a select() event anymore. Also move the call to select() after g_poll(), this will improve latency as we don't have to go through two os_host_main_loop_wait() calls to detect a socket event. Reviewed-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Fabien Chouteau <chouteau@adacore.com> Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Diffstat (limited to 'main-loop.c')
-rw-r--r--main-loop.c27
1 files changed, 13 insertions, 14 deletions
diff --git a/main-loop.c b/main-loop.c
index 54f38ae1ae..6f52ac39bc 100644
--- a/main-loop.c
+++ b/main-loop.c
@@ -330,7 +330,7 @@ void qemu_fd_register(int fd)
static int os_host_main_loop_wait(uint32_t timeout)
{
GMainContext *context = g_main_context_default();
- int ret, i;
+ int select_ret, g_poll_ret, ret, i;
PollingEntry *pe;
WaitObjects *w = &wait_objects;
gint poll_timeout;
@@ -345,13 +345,6 @@ static int os_host_main_loop_wait(uint32_t timeout)
return ret;
}
- if (nfds >= 0) {
- ret = select(nfds + 1, &rfds, &wfds, &xfds, &tv0);
- if (ret != 0) {
- timeout = 0;
- }
- }
-
g_main_context_prepare(context, &max_priority);
n_poll_fds = g_main_context_query(context, max_priority, &poll_timeout,
poll_fds, ARRAY_SIZE(poll_fds));
@@ -367,9 +360,9 @@ static int os_host_main_loop_wait(uint32_t timeout)
}
qemu_mutex_unlock_iothread();
- ret = g_poll(poll_fds, n_poll_fds + w->num, poll_timeout);
+ g_poll_ret = g_poll(poll_fds, n_poll_fds + w->num, poll_timeout);
qemu_mutex_lock_iothread();
- if (ret > 0) {
+ if (g_poll_ret > 0) {
for (i = 0; i < w->num; i++) {
w->revents[i] = poll_fds[n_poll_fds + i].revents;
}
@@ -384,12 +377,18 @@ static int os_host_main_loop_wait(uint32_t timeout)
g_main_context_dispatch(context);
}
- /* If an edge-triggered socket event occurred, select will return a
- * positive result on the next iteration. We do not need to do anything
- * here.
+ /* Call select after g_poll to avoid a useless iteration and therefore
+ * improve socket latency.
*/
- return ret;
+ if (nfds >= 0) {
+ select_ret = select(nfds + 1, &rfds, &wfds, &xfds, &tv0);
+ if (select_ret != 0) {
+ timeout = 0;
+ }
+ }
+
+ return select_ret || g_poll_ret;
}
#endif