unnecessary removal of guest console characters

Hi all,
currently when max_capacity is set in xenconsoled and the buffer if completely filled then some characters in the buffer are discarded (even though the rate limitation makes this unlikely to happen).
A better way to handle this scenario would be to stop reading from the ring until the buffer has some free space.
I am attaching a simple patch that does exactly this.
Best Regards,

Stefano Stabellini

Signed-off-by Stefano Stabellini <[email protected]>

diff -r 082d3886fded tools/console/daemon/io.c

tools/console/daemon/io.c Fri Apr 11 14:12:51 2008 +0100 renamed from tools/console/daemon/io.c Fri Apr 11 09:14:03 2008 +0100 +11 -18

@@ -200,21 +200,6 @@ static void buffer_append(struct domain
     "on domain %d: %d (%s)\n",
     dom->domid, errno, strerror(errno));
 }
-
- if (buffer->max_capacity &&
-     buffer->size > buffer->max_capacity) {
-  /* Discard the middle of the data. */
-
-  size_t over = buffer->size - buffer->max_capacity;
-  char *maxpos = buffer->data + buffer->max_capacity;
-
-  memmove(maxpos - over, maxpos, over);
-  buffer->data = realloc(buffer->data, buffer->max_capacity);
-  buffer->size = buffer->capacity = buffer->max_capacity;
-
-  if (buffer->consumed > buffer->max_capacity - over)
-   buffer->consumed = buffer->max_capacity - over;
- }
 }
 
 static bool buffer_empty(struct buffer *buffer)

@@ -228,6 +213,11 @@ static void buffer_advance(struct buffer
 if (buffer->consumed == buffer->size) {
 buffer->consumed = 0;
 buffer->size = 0;
+  if (buffer->max_capacity &&
+      buffer->capacity > buffer->max_capacity) {
+   buffer->data = realloc(buffer->data, buffer->max_capacity);
+   buffer->capacity = buffer->max_capacity;
+  }
 }
 }
 

@@ -1005,9 +995,12 @@ void handle_io(void)
   d->next_period < next_timeout)
 next_timeout = d->next_period;
 } else if (d->xce_handle != -1) {
-    int evtchn_fd = xc_evtchn_fd(d->xce_handle);
-    FD_SET(evtchn_fd, &readfds);
-    max_fd = MAX(evtchn_fd, max_fd);
+    if (!d->buffer.max_capacity ||
+        d->buffer.size < d->buffer.max_capacity) {
+     int evtchn_fd = xc_evtchn_fd(d->xce_handle);
+     FD_SET(evtchn_fd, &readfds);
+     max_fd = MAX(evtchn_fd, max_fd);
+    }
 }
 
 if (d->master_fd != -1) {

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Buffering happens in the daemon, not in the inter-domain ring. If we switch to your strategy then overflow leads to the guest hanging rather than characters being dropped.

Quoted 3 lines · Keir Fraser, le Fri 11 Apr 2008 16:03:57 +0100, a écrit :

Buffering happens in the daemon, not in the inter-domain ring. If we switch to your strategy then overflow leads to the guest hanging rather than characters being dropped.

The guest itself doesn't hang, the application inside the guest that produces the output does hang, and that's what I would expect if I am not looking at it, rather than the risk of loosing data because of a non-end-to-end flow control.

Samuel