Re: [SeaBIOS] [PATCH v2] qemu: fast boot when linuxboot optionrom is used

[Stefano Garzarella]

On Fri, Nov 30, 2018 at 2:06 PM Gerd Hoffmann  wrote:
> Hi,
> > > Note: All these tests are done with CONFIG_DEBUG_LEVEL=0, using
> > > CONFIG_DEBUG_LEVEL=1 the boot time grows up to 24 ms, maybe we should
> > > put CONFIG_DEBUG_LEVEL=0 in the SeaBIOS configuration used in QEMU.
> >
> > I think the main seabios binary should have CONFIG_DEBUG_LEVEL=1 as it
> > helps with debug reports.  I suppose an additional binary could be
> > made for those looking for the fastest possible speed.  (The sole cost
> > of the debugging is the additional hardware accesses that results from
> > those debug messages.)
>
> The qemu debugcon (CONFIG_DEBUG_IO) is detecable at runtime, it returns
> 0xe9 on port reads.  So we should be able to skip that too.  IIRC it
> isn't *that* straightforward as seabios is initially mapped read/only so
> a simple probe-on-first-putchar, then cache the result in a variable
> doesn't work.  We could probe after make_bios_writable though which
> should still avoid printing most of the messages.

Great! I just tried the patch below and it works as you said. If I
don't have debugcon in QEMU the outs are avoided and the speed is not
bad (11.4 ms).
If I have debugcon (iobase=0x402), I can see the output.

Do you think can be an acceptable trade-off?

Thanks.
Stefano

diff --git a/src/fw/paravirt.c b/src/fw/paravirt.c
index 0770c47..31c080e 100644
--- a/src/fw/paravirt.c
+++ b/src/fw/paravirt.c
@@ -121,6 +121,10 @@ qemu_preinit(void)
         kvm_detect();
     }

+    // Detect qemu debugcon
+    if (inb(GET_GLOBAL(DebugOutputPort)) != QEMU_DEBUGCON_READBACK)
+        DebugOutputPort = 0;
+
     // On emulators, get memory size from nvram.
     u32 rs = ((rtc_read(CMOS_MEM_EXTMEM2_LOW) << 16)
               | (rtc_read(CMOS_MEM_EXTMEM2_HIGH) << 24));
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..4e0f2ac 100644
--- a/src/hw/serialio.c
+++ b/src/hw/serialio.c
@@ -107,7 +107,7 @@ u16 DebugOutputPort VARFSEG = 0x402;
 void
 qemu_debug_putc(char c)
 {
-    if (CONFIG_DEBUG_IO && runningOnQEMU())
+    if (CONFIG_DEBUG_IO && runningOnQEMU() && GET_GLOBAL(DebugOutputPort))
         // Send character to debug port.
         outb(c, GET_GLOBAL(DebugOutputPort));
 }
> cheers,
>   Gerd