[SeaBIOS] [PATCH] Preserve Xen DebugOutputPort

[Jason Andryuk]

xen_preinit() runs early and changes DebugOutputPort.  qemu_preinit() runs
soon after.  inb on DebugOutputPort doesn't work on Xen, so the check
will always fail and DebugOutputPort will be cleared to 0 disabling
output.

Quick exit the function when running on Xen to preserve the modified
DebugOutputPort.

Signed-off-by: Jason Andryuk 
---
 src/hw/serialio.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/hw/serialio.c b/src/hw/serialio.c
index 3163344..c6297bc 100644
--- a/src/hw/serialio.c
+++ b/src/hw/serialio.c
@@ -106,6 +106,10 @@ u16 DebugOutputPort VARFSEG = 0x402;
 void
 qemu_debug_preinit(void)
 {
+    /* Xen already overrode DebugOutputPort in xen_preinit(). */
+    if (runningOnXen())
+        return;
+
     /* Check if the QEMU debug output port is active */
     if (CONFIG_DEBUG_IO &&
         inb(GET_GLOBAL(DebugOutputPort)) != QEMU_DEBUGCON_READBACK)