char: Throttle when host connection is down#

When the host-side connection goes down, throttle the virtio-serial bus
and later unthrottle when a connection gets established.  This helps
prevent any lost IO (guest->host) while the host connection was down.

Bugzilla: 621484

This commit actually helps the bug mentioned above as no writes will now
get lost because of the throttling done here.  With just the patches
sent earlier for that bug, one write will end up getting lost in the
worst case (host d/c, guest write, host connect).

Signed-off-by: Amit Shah <amit.shah@redhat.com>

1 files changed, 14 insertions, 0 deletions

diff --git a/qemu-char.c b/qemu-char.c
index eff4ff268..1cc36c57b 100644
--- a/qemu-char.c
+++ b/qemu-char.c
@@ -140,6 +140,9 @@ static void qemu_chr_fire_open_event(void *opaque)
 {
     CharDriverState *s = opaque;
     qemu_chr_be_event(s, CHR_EVENT_OPENED);
+    if (s->write_blocked) {
+        char_write_unblocked(s);
+    }
     qemu_free_timer(s->open_timer);
     s->open_timer = NULL;
 }
@@ -2249,6 +2252,17 @@ static int tcp_chr_write(CharDriverState *chr, const uint8_t *buf, int len)
         ret = send_all(chr, s->fd, buf, len);
         if (ret == -1 && errno == EPIPE) {
             tcp_closed(chr);
+
+            if (chr->chr_enable_write_fd_handler && chr->chr_write_unblocked) {
+                /*
+                 * Since we haven't written out anything, let's say
+                 * we're throttled.  This will prevent any output from
+                 * the guest getting lost if host-side chardev goes
+                 * down.  Unthrottle when we re-connect.
+                 */
+                chr->write_blocked = true;
+                return 0;
+            }
         }
         return ret;
     } else {