From: Chris Lalancette <clalance@redhat.com>
Date: Tue, 13 Oct 2009 09:50:06 +0200
Subject: [xen] allow booting with broken serial hardware
Message-id: 4AD4312E.8060508@redhat.com
O-Subject: [RHEL5.5 PATCH v2]: Allow Xen to boot with broken serial hardware
Bugzilla: 518338
RH-Acked-by: Markus Armbruster <armbru@redhat.com>
RH-Acked-by: Don Dutile <ddutile@redhat.com>
RH-Acked-by: Andrew Jones <drjones@redhat.com>

All,
     We've found that the serial console on a new piece of vendor hardware is
broken.  In particular, it isn't draining the TX queue as promptly as it should
be.  While this is a hardware problem, it causes undesirable behavior in the Xen
kernel; namely, the Xen kernel takes ages to boot.
     The problem comes down to a broken piece of code in the
guest_console_write() function.  It was checking for the case where the serial
TX buffer was half-full; if it was half-full (or greater), it would just
continually relax the processor until the TX buffer cleared out.  With normal
serial hardware, this is no problem, but with the broken serial hardware this
would cause lots of delays.
     The fix is two-fold.  First, upstream has recently added some tx_quench
stuff, which effectively drops characters if the tx buffer is full.  Second, we
need to remove the check that does the checking of the TX buffer half full.
     With these fixes in place, I was able to successfully reboot the problem
machine many times without problems.  The customer also reports that the patch
fixes the problem on their hardware.
     This is a backport of xen-unstable.hg c/s 17851 and 20217.  Version 2 of
this patch deviates slightly from upstream in that we make sure that guests
can't flood the serial console (thanks to Markus for the thoughts/review here).

This should resolve BZ 518338.  Please review and ACK.

--
Chris Lalancette

diff --git a/common/keyhandler.c b/common/keyhandler.c
index 0861f85..c8e05bc 100644
--- a/common/keyhandler.c
+++ b/common/keyhandler.c
@@ -36,10 +36,10 @@ static void keypress_softirq(void)
 {
     keyhandler_t *h;
     unsigned char key = keypress_key;
-    console_start_log_everything();
+    console_start_sync();
     if ( (h = key_table[key].u.handler) != NULL )
         (*h)(key);
-    console_end_log_everything();
+    console_end_sync();
 }
 
 void handle_keypress(unsigned char key, struct cpu_user_regs *regs)
@@ -48,10 +48,10 @@ void handle_keypress(unsigned char key, struct cpu_user_regs *regs)
 
     if ( !in_irq() || (key_table[key].flags & KEYHANDLER_IRQ_CALLBACK) )
     {
-        console_start_log_everything();
+        console_start_sync();
         if ( (h = key_table[key].u.irq_handler) != NULL )
             (*h)(key, regs);
-        console_end_log_everything();
+        console_end_sync();
     }
     else
     {
diff --git a/drivers/char/console.c b/drivers/char/console.c
index de35126..22b2e6a 100644
--- a/drivers/char/console.c
+++ b/drivers/char/console.c
@@ -313,18 +313,16 @@ static long guest_console_write(XEN_GUEST_HANDLE(char) buffer, int count)
 
     while ( count > 0 )
     {
-        while ( serial_tx_space(sercon_handle) < (SERIAL_TXBUFSZ / 2) )
-        {
-            if ( hypercall_preempt_check() )
-                break;
-            cpu_relax();
-        }
-
         if ( hypercall_preempt_check() )
             return hypercall_create_continuation(
                 __HYPERVISOR_console_io, "iih",
                 CONSOLEIO_write, count, buffer);
 
+	if ( serial_tx_space(sercon_handle) < (SERIAL_TXBUFSZ / 2) )
+	{
+	    return 0;
+	}
+
         kcount = min_t(int, count, sizeof(kbuf)-1);
         if ( copy_from_guest(kbuf, buffer, kcount) )
             return -EFAULT;
@@ -587,16 +585,6 @@ void __init console_endboot(void)
     switch_serial_input();
 }
 
-void console_start_log_everything(void)
-{
-    atomic_inc(&print_everything);
-}
-
-void console_end_log_everything(void)
-{
-    atomic_dec(&print_everything);
-}
-
 void console_force_unlock(void)
 {
     spin_lock_init(&console_lock);
@@ -611,14 +599,14 @@ void console_force_lock(void)
 
 void console_start_sync(void)
 {
-    console_start_log_everything();
+    atomic_inc(&print_everything);
     serial_start_sync(sercon_handle);
 }
 
 void console_end_sync(void)
 {
     serial_end_sync(sercon_handle);
-    console_end_log_everything();
+    atomic_dec(&print_everything);
 }
 
 void console_putc(char c)
diff --git a/drivers/char/serial.c b/drivers/char/serial.c
index bb196ac..6c75932 100644
--- a/drivers/char/serial.c
+++ b/drivers/char/serial.c
@@ -3,7 +3,7 @@
  * 
  * Framework for serial device drivers.
  * 
- * Copyright (c) 2003-2005, K A Fraser
+ * Copyright (c) 2003-2008, K A Fraser
  */
 
 #include <xen/config.h>
@@ -81,13 +81,21 @@ void serial_tx_interrupt(struct serial_port *port, struct cpu_user_regs *regs)
 
 static void __serial_putc(struct serial_port *port, char c)
 {
-    int i;
-
     if ( (port->txbuf != NULL) && !port->sync )
     {
         /* Interrupt-driven (asynchronous) transmitter. */
+        if ( port->tx_quench )
+        {
+            /* Buffer filled and we are dropping characters. */
+            if ( (port->txbufp - port->txbufc) > (SERIAL_TXBUFSZ / 2) )
+                return;
+            port->tx_quench = 0;
+        }
+
         if ( (port->txbufp - port->txbufc) == SERIAL_TXBUFSZ )
         {
+#ifdef SERIAL_NEVER_DROP_CHARS
+            int i;
             /* Buffer is full: we spin, but could alternatively drop chars. */
             while ( !port->driver->tx_empty(port) )
                 cpu_relax();
@@ -95,6 +103,10 @@ static void __serial_putc(struct serial_port *port, char c)
                 port->driver->putc(
                     port, port->txbuf[MASK_SERIAL_TXBUF_IDX(port->txbufc++)]);
             port->txbuf[MASK_SERIAL_TXBUF_IDX(port->txbufp++)] = c;
+#else
+            /* Buffer is full: drop characters until buffer is half empty. */
+            port->tx_quench = 1;
+#endif
         }
         else if ( ((port->txbufp - port->txbufc) == 0) &&
                   port->driver->tx_empty(port) )
diff --git a/include/xen/console.h b/include/xen/console.h
index d6c0510..629612a 100644
--- a/include/xen/console.h
+++ b/include/xen/console.h
@@ -26,9 +26,6 @@ void console_force_lock(void);
 void console_start_sync(void);
 void console_end_sync(void);
 
-void console_start_log_everything(void);
-void console_end_log_everything(void);
-
 /*
  * Steal output from the console. Returns +ve identifier, else -ve error.
  * Takes the handle of the serial line to steal, and steal callback function.
diff --git a/include/xen/serial.h b/include/xen/serial.h
index 75a07c9..926d48f 100644
--- a/include/xen/serial.h
+++ b/include/xen/serial.h
@@ -3,7 +3,7 @@
  * 
  * Framework for serial device drivers.
  * 
- * Copyright (c) 2003-2005, K A Fraser
+ * Copyright (c) 2003-2008, K A Fraser
  */
 
 #ifndef __XEN_SERIAL_H__
@@ -34,6 +34,7 @@ struct serial_port {
     /* Transmit data buffer (interrupt-driven uart). */
     char               *txbuf;
     unsigned int        txbufp, txbufc;
+    bool_t              tx_quench;
     /* Force synchronous transmit. */
     int                 sync;
     /* Receiver callback functions (asynchronous receivers). */