From c2ddb156db52808a89d91edf5bb1449f226d3ecd Mon Sep 17 00:00:00 2001
Message-Id: <c2ddb156db52808a89d91edf5bb1449f226d3ecd.1354903384.git.crobinso@redhat.com>
In-Reply-To: <9f0944a25bc1094fa7a74ac9df14e184e2c5c82d.1354903384.git.crobinso@redhat.com>
References: <9f0944a25bc1094fa7a74ac9df14e184e2c5c82d.1354903384.git.crobinso@redhat.com>
From: Amit Shah <amit.shah@redhat.com>
Date: Mon, 21 Mar 2011 22:05:10 +0100
Subject: [PATCH] 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>
Signed-off-by: Cole Robinson <crobinso@redhat.com>
---
 qemu-char.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/qemu-char.c b/qemu-char.c
index e7cd42a..5c71f0c 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 {
-- 
1.8.0