Re: [Qemu-devel] [RFC 0/3] qemu-char: Add poll timeouts for character backends

[Paolo Bonzini <pbonzini@redhat.com>]

On 10/24/2014 10:13 AM, Heinz Graalfs wrote:
> On s390 one can observe system hang situations wrt console input when
> using 'dataplane=on'.
> 
> dataplane processing causes an inactive main thread and an active
> dataplane thread.
> 
> When a character backend descriptor disappears from the main thread's
> poll() descriptor array (when can_read() returns 0) it happens that it
> will never reappear in the poll() array due to missing poll() interrupts.
> 
> The following patches fix observed hangs on s390 and provide a means
> to avoid potential hangs in other backends/frontends.

I think all you need is a simple

    qemu_notify_event();

call when can_read can go from 0 to 1, for example just before
get_console_data returns (for hw/char/sclpconsole-lm.c).

By the way, for hw/char/sclpconsole-lm.c I'm not sure what happens if
scon->length == SIZE_CONSOLE_BUFFER.  You cannot read, so you cannot
generate an event, and you cannot reset scon->length because you cannot
generate an event.  I think something like this is needed:

diff --git a/hw/char/sclpconsole-lm.c b/hw/char/sclpconsole-lm.c
index 80dd0a9..c61b77b 100644
--- a/hw/char/sclpconsole-lm.c
+++ b/hw/char/sclpconsole-lm.c
@@ -61,10 +61,9 @@ static int chr_can_read(void *opaque)

     if (scon->event.event_pending) {
         return 0;
-    } else if (SIZE_CONSOLE_BUFFER - scon->length) {
+    } else {
         return 1;
     }
-    return 0;
 }

 static void chr_read(void *opaque, const uint8_t *buf, int size)
@@ -78,6 +77,10 @@ static void chr_read(void *opaque, const uint8_t
*buf, int size)
         sclp_service_interrupt(0);
         return;
     }
+    if (scon->length == SIZE_CONSOLE_BUFFER) {
+        /* Eat the character, but still process CR and LF.  */
+        return;
+    }
     scon->buf[scon->length] = *buf;
     scon->length += 1;
     if (scon->echo) {

Paolo