[SeaBIOS] [PATCH] qemu: avoid debug prints if debugcon is not enabled

Stefano Garzarella sgarzare at redhat.com
Sun Dec 2 14:10:13 CET 2018


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 at redhat.com>
---
 src/fw/paravirt.h |  3 +++
 src/hw/serialio.c | 16 ++++++++++++++--
 src/hw/serialio.h |  1 +
 src/post.c        |  3 +++
 4 files changed, 21 insertions(+), 2 deletions(-)

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 @@ static inline int runningOnKVM(void) {
 // 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..d9acd3c 100644
--- a/src/hw/serialio.c
+++ b/src/hw/serialio.c
@@ -103,11 +103,23 @@ serial_debug_flush(void)
 
 u16 DebugOutputPort VARFSEG = 0x402;
 
+void
+qemu_debug_postram_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())
+    u16 port;
+
+    if (CONFIG_DEBUG_IO && runningOnQEMU() &&
+        (port = GET_GLOBAL(DebugOutputPort)))
         // 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..31fbea3 100644
--- a/src/hw/serialio.h
+++ b/src/hw/serialio.h
@@ -24,6 +24,7 @@ void serial_debug_preinit(void);
 void serial_debug_putc(char c);
 void serial_debug_flush(void);
 extern u16 DebugOutputPort;
+void qemu_debug_postram_preinit(void);
 void qemu_debug_putc(char c);
 
 #endif // serialio.h
diff --git a/src/post.c b/src/post.c
index f93106a..650a314 100644
--- a/src/post.c
+++ b/src/post.c
@@ -332,6 +332,9 @@ handle_post(void)
     // Allow writes to modify bios area (0xf0000)
     make_bios_writable();
 
+    // Setup QEMU debug output port
+    qemu_debug_postram_preinit();
+
     // Now that memory is read/writable - start post process.
     dopost();
 }
-- 
2.19.2




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