qemu: avoid debug prints if debugcon is not enabled

In order to speed up the boot phase, we can check the QEMU
debugcon device, and disable the writes if it is not recognized.

This patch allow us to save around 10 msec (time measured
between SeaBIOS entry point and "linuxboot" entry point)
when CONFIG_DEBUG_LEVEL=1 and debugcon is not enabled.

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
diff --git a/src/fw/paravirt.c b/src/fw/paravirt.c
index 0770c47..4fcd8f5 100644
--- a/src/fw/paravirt.c
+++ b/src/fw/paravirt.c
@@ -75,6 +75,9 @@
     if (!CONFIG_QEMU_HARDWARE)
         return;
 
+    // Setup QEMU debug output port
+    qemu_debug_preinit();
+
     // check northbridge @ 00:00.0
     u16 v = pci_config_readw(0, PCI_VENDOR_ID);
     if (v == 0x0000 || v == 0xffff)
diff --git a/src/fw/paravirt.h b/src/fw/paravirt.h
index a14d83e..f7e1d4c 100644
--- a/src/fw/paravirt.h
+++ b/src/fw/paravirt.h
@@ -49,6 +49,9 @@
 // QEMU_CFG_DMA ID bit
 #define QEMU_CFG_VERSION_DMA    2
 
+// QEMU debugcon read value
+#define QEMU_DEBUGCON_READBACK  0xe9
+
 int qemu_cfg_enabled(void);
 int qemu_cfg_dma_enabled(void);
 void qemu_preinit(void);
diff --git a/src/hw/serialio.c b/src/hw/serialio.c
index 319a85c..3163344 100644
--- a/src/hw/serialio.c
+++ b/src/hw/serialio.c
@@ -103,11 +103,23 @@
 
 u16 DebugOutputPort VARFSEG = 0x402;
 
+void
+qemu_debug_preinit(void)
+{
+    /* Check if the QEMU debug output port is active */
+    if (CONFIG_DEBUG_IO &&
+        inb(GET_GLOBAL(DebugOutputPort)) != QEMU_DEBUGCON_READBACK)
+        DebugOutputPort = 0;
+}
+
 // Write a character to the special debugging port.
 void
 qemu_debug_putc(char c)
 {
-    if (CONFIG_DEBUG_IO && runningOnQEMU())
+    if (!CONFIG_DEBUG_IO || !runningOnQEMU())
+        return;
+    u16 port = GET_GLOBAL(DebugOutputPort);
+    if (port)
         // Send character to debug port.
-        outb(c, GET_GLOBAL(DebugOutputPort));
+        outb(c, port);
 }
diff --git a/src/hw/serialio.h b/src/hw/serialio.h
index 88296fe..81fed30 100644
--- a/src/hw/serialio.h
+++ b/src/hw/serialio.h
@@ -24,6 +24,7 @@
 void serial_debug_putc(char c);
 void serial_debug_flush(void);
 extern u16 DebugOutputPort;
+void qemu_debug_preinit(void);
 void qemu_debug_putc(char c);
 
 #endif // serialio.h