[syslinux] [PATCH 0/1] dprintf: add debug console support

[Jonathan Boeing]

On Sun, 8 Feb 2015 17:41:58 +0100
Geert Stappers via Syslinux <syslinux at zytor.com> wrote:

> On Sun, Feb 08, 2015 at 09:13:02AM -0700, Jonathan Boeing via
> Syslinux wrote:
> > This patch adds support for printing messages through a debug
> > console.  QEMU, for example, supports this through the debugcon
> > facility.  The benefit is that it's *much* faster than printing
> > over a serial port.
> > 
> > To print to I/O port 0x402 (the default used by SeaBIOS and OVMF),
> > add "-DDEBUG_IO_PORT=0x402 -DCORE_DEBUG=1" to the build CFLAGS.
> > 
> > To enable a debug console in QEMU that listens to I/O port 0x402
> > and prints to stdio, add "-debugcon stdio -global
> > isa-debugcon.iobase=0x402" to the QEMU command line.
> 
> 
> When enabled, what will the effect on real hardware without the
> diagnostic hardware ?
> 
> When enabled, what will the effect on real hardware ?
> 
> What (real) diagnostic hardware is adviced?
> 

This doesn't support real hardware[1]; it's a VM-only thing.

If run on real hardware, it shouldn't have any effect[2] as long as it's
writing to an unused I/O port.  Port 0x402 seems to be unused (based on
some google searching) outside of debug consoles.  Bochs has a list at
http://bochs.sourceforge.net/techspec/PORTS.LST that shows it's in the
EISA range, but not specifically used.

So, it should be safe on real hardware, but I'd recommend it only for
testing and development on a VM.

Also, syslinux works correctly if it's enabled and run on a VM without a
debug console.  The writes are just silently discarded.



[1] Specialized (read expensive) CPU debugging hardware could capture
these writes.  I'm not sure if Intel System Studio can capture them,
but it's still $2399 for the JTAG-equipped version.

[2] It's CPU implementation specific, but it shouldn't have any visible
effect.

Regards,
Jonathan Boeing