Patchwork debugcon: support for debugging consoles (e.g. Bochs port 0xe9)

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Submitter H. Peter Anvin
Date Dec. 29, 2009, 9:51 p.m.
Message ID <1262123496-12007-1-git-send-email-hpa@linux.intel.com>
Download mbox | patch
Permalink /patch/41896/
State New
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Comments

H. Peter Anvin - Dec. 29, 2009, 9:51 p.m.
Add generic support for debugging consoles (simple I/O ports which
when written to cause debugging output to be written to a target.)
The current implementation matches Bochs' port 0xe9, allowing the same
debugging code to be used for both Bochs and Qemu.

There is no vm state associated with the debugging port, simply
because it has none -- the entire interface is a single, stateless,
write-only port.

Most of the code was cribbed from the serial port driver.

v2: removed non-ISA variants (they can be introduced when/if someone
wants them, using code from the serial port); added configurable
readback (Bochs returns 0xe9 on a read from this register, mimic that
by default)  This retains the apparently somewhat controversial user
friendly option, however.

v3: reimplemented the user friendly option as a synthetic option
("-debugcon foo" basically ends up being a parser-level shorthand for
"-chardev stdio,id=debugcon -device isa-debugcon,chardev=debugcon") --
this dramatically reduced the complexity while keeping the same level
of user friendliness.

v4: spaces, not tabs.

v5: update to match current top of tree.  Calling qemu_chr_open()
already during parsing no longer works; defer until we are parsing the
other console-like devices.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
---
 Makefile.target |    2 +-
 hw/debugcon.c   |  107 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 qemu-options.hx |   11 ++++++
 vl.c            |   23 ++++++++++++
 4 files changed, 142 insertions(+), 1 deletions(-)
 create mode 100644 hw/debugcon.c
Kevin O'Connor - Dec. 30, 2009, 4:49 p.m.
On Tue, Dec 29, 2009 at 01:51:36PM -0800, H. Peter Anvin wrote:
> Add generic support for debugging consoles (simple I/O ports which
> when written to cause debugging output to be written to a target.)
> The current implementation matches Bochs' port 0xe9, allowing the same
> debugging code to be used for both Bochs and Qemu.
> 
> There is no vm state associated with the debugging port, simply
> because it has none -- the entire interface is a single, stateless,
> write-only port.
> 
> Most of the code was cribbed from the serial port driver.

Hi,

SeaBIOS writes debugging info to port 0x0402.  Unfortunately, qemu has
to be recompiled in order to display this info.  Will your patch
enable one to get at the 0x0402 data without recompiling?

-Kevin
H. Peter Anvin - Jan. 2, 2010, 3:01 a.m.
On 12/30/2009 08:49 AM, Kevin O'Connor wrote:
> On Tue, Dec 29, 2009 at 01:51:36PM -0800, H. Peter Anvin wrote:
>> Add generic support for debugging consoles (simple I/O ports which
>> when written to cause debugging output to be written to a target.)
>> The current implementation matches Bochs' port 0xe9, allowing the same
>> debugging code to be used for both Bochs and Qemu.
>>
>> There is no vm state associated with the debugging port, simply
>> because it has none -- the entire interface is a single, stateless,
>> write-only port.
>>
>> Most of the code was cribbed from the serial port driver.
> 
> Hi,
> 
> SeaBIOS writes debugging info to port 0x0402.  Unfortunately, qemu has
> to be recompiled in order to display this info.  Will your patch
> enable one to get at the 0x0402 data without recompiling?
> 

Yes.

	-hpa
H. Peter Anvin - Jan. 2, 2010, 3:02 a.m.
On 12/30/2009 08:49 AM, Kevin O'Connor wrote:
> 
> Hi,
> 
> SeaBIOS writes debugging info to port 0x0402.  Unfortunately, qemu has
> to be recompiled in order to display this info.  Will your patch
> enable one to get at the 0x0402 data without recompiling?
> 
> -Kevin
> 

Incidentally, it's somewhat unusual choice of ports... port 0x80 is the
normal port for BIOSes to display debugging information on.

	-hpa
Kevin O'Connor - Jan. 2, 2010, 5:53 p.m.
On Fri, Jan 01, 2010 at 07:02:59PM -0800, H. Peter Anvin wrote:
> On 12/30/2009 08:49 AM, Kevin O'Connor wrote:
> > SeaBIOS writes debugging info to port 0x0402.  Unfortunately, qemu has
> > to be recompiled in order to display this info.  Will your patch
> > enable one to get at the 0x0402 data without recompiling?
> 
> Incidentally, it's somewhat unusual choice of ports... port 0x80 is the
> normal port for BIOSes to display debugging information on.

Port 0x0402 is a carry over from Bochs bios.

Port 0x80 isn't generally used to emit a stream of ascii data, and it
can be used by other programs besides the bios.  I'd guess that's why
Bochs chose a different port.

-Kevin
H. Peter Anvin - Jan. 2, 2010, 6:56 p.m.
On 01/02/2010 09:53 AM, Kevin O'Connor wrote:
> On Fri, Jan 01, 2010 at 07:02:59PM -0800, H. Peter Anvin wrote:
>> On 12/30/2009 08:49 AM, Kevin O'Connor wrote:
>>> SeaBIOS writes debugging info to port 0x0402.  Unfortunately, qemu has
>>> to be recompiled in order to display this info.  Will your patch
>>> enable one to get at the 0x0402 data without recompiling?
>>
>> Incidentally, it's somewhat unusual choice of ports... port 0x80 is the
>> normal port for BIOSes to display debugging information on.
> 
> Port 0x0402 is a carry over from Bochs bios.
> 
> Port 0x80 isn't generally used to emit a stream of ascii data, and it
> can be used by other programs besides the bios.  I'd guess that's why
> Bochs chose a different port.
> 
> -Kevin

Makes sense.  Either way, it works with my patch looking something like:

	-chardev stdio,id=seabios \
	-device isa-debugcon,iobase=0x402,chardev=seabios

... should do the job.

	-hpa
Fran├žois Revol - Jan. 3, 2010, 2:13 a.m.
> Makes sense.  Either way, it works with my patch looking something 
> like:
> 
> 	-chardev stdio,id=seabios \
> 	-device isa-debugcon,iobase=0x402,chardev=seabios
> 
> ... should do the job.

Btw,
I once added another method of debug output to ZETA, that was using the 
low-level protocol used by laplink (nibble mode). I needed this for a 
laptop that didn't have a serial port but still had a // one. This 
allowed bidirectionnal debugging (including sending kernel debugger 
commands) with a null-printer cable.
I also wrote code to support it in Haiku, though I'm not sure I've 
added the receiving driver yet.
See:
http://dev.haiku-os.org/browser/haiku/trunk/src/add-ons/kernel/debugger/laplinkll

I don't know if the parallel port emulation in QEMU supports dumping 
bytes at nibble mode yet, it might be interesting to add this anyway.
Though it should also be possible to just use 8bit mode and dump data 
straight to it as with the 0x402 port.

Fran├žois.
Gerd Hoffmann - Jan. 4, 2010, 2:13 p.m.
On 12/29/09 22:51, H. Peter Anvin wrote:
> Add generic support for debugging consoles (simple I/O ports which
> when written to cause debugging output to be written to a target.)
> The current implementation matches Bochs' port 0xe9, allowing the same
> debugging code to be used for both Bochs and Qemu.

Looks good to me.

Acked-by: Gerd Hoffmann <kraxel@redhat.com>

cheers,
   Gerd
Anthony Liguori - Jan. 8, 2010, 4:36 p.m.
On 12/29/2009 03:51 PM, H. Peter Anvin wrote:
> Add generic support for debugging consoles (simple I/O ports which
> when written to cause debugging output to be written to a target.)
> The current implementation matches Bochs' port 0xe9, allowing the same
> debugging code to be used for both Bochs and Qemu.
>
> There is no vm state associated with the debugging port, simply
> because it has none -- the entire interface is a single, stateless,
> write-only port.
>
> Most of the code was cribbed from the serial port driver.
>
> v2: removed non-ISA variants (they can be introduced when/if someone
> wants them, using code from the serial port); added configurable
> readback (Bochs returns 0xe9 on a read from this register, mimic that
> by default)  This retains the apparently somewhat controversial user
> friendly option, however.
>
> v3: reimplemented the user friendly option as a synthetic option
> ("-debugcon foo" basically ends up being a parser-level shorthand for
> "-chardev stdio,id=debugcon -device isa-debugcon,chardev=debugcon") --
> this dramatically reduced the complexity while keeping the same level
> of user friendliness.
>
> v4: spaces, not tabs.
>
> v5: update to match current top of tree.  Calling qemu_chr_open()
> already during parsing no longer works; defer until we are parsing the
> other console-like devices.
>
> Signed-off-by: H. Peter Anvin<hpa@linux.intel.com>
>    

Applied.  Thanks.

Regards,

Anthony Liguori
> ---
>   Makefile.target |    2 +-
>   hw/debugcon.c   |  107 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
>   qemu-options.hx |   11 ++++++
>   vl.c            |   23 ++++++++++++
>   4 files changed, 142 insertions(+), 1 deletions(-)
>   create mode 100644 hw/debugcon.c
>
> diff --git a/Makefile.target b/Makefile.target
> index 7c1f30c..0a803ef 100644
> --- a/Makefile.target
> +++ b/Makefile.target
> @@ -194,7 +194,7 @@ obj-i386-y += fdc.o mc146818rtc.o serial.o i8259.o i8254.o pcspk.o pc.o
>   obj-i386-y += cirrus_vga.o apic.o ioapic.o parallel.o acpi.o piix_pci.o
>   obj-i386-y += usb-uhci.o vmmouse.o vmport.o vmware_vga.o hpet.o
>   obj-i386-y += device-hotplug.o pci-hotplug.o smbios.o wdt_ib700.o
> -obj-i386-y += ne2000-isa.o
> +obj-i386-y += ne2000-isa.o debugcon.o
>
>   # shared objects
>   obj-ppc-y = ppc.o ide/core.o ide/qdev.o ide/isa.o ide/pci.o ide/macio.o
> diff --git a/hw/debugcon.c b/hw/debugcon.c
> new file mode 100644
> index 0000000..d549091
> --- /dev/null
> +++ b/hw/debugcon.c
> @@ -0,0 +1,107 @@
> +/*
> + * QEMU Bochs-style debug console ("port E9") emulation
> + *
> + * Copyright (c) 2003-2004 Fabrice Bellard
> + * Copyright (c) 2008 Citrix Systems, Inc.
> + * Copyright (c) Intel Corporation; author: H. Peter Anvin
> + *
> + * Permission is hereby granted, free of charge, to any person obtaining a copy
> + * of this software and associated documentation files (the "Software"), to deal
> + * in the Software without restriction, including without limitation the rights
> + * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
> + * copies of the Software, and to permit persons to whom the Software is
> + * furnished to do so, subject to the following conditions:
> + *
> + * The above copyright notice and this permission notice shall be included in
> + * all copies or substantial portions of the Software.
> + *
> + * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
> + * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
> + * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
> + * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
> + * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
> + * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
> + * THE SOFTWARE.
> + */
> +
> +#include "hw.h"
> +#include "qemu-char.h"
> +#include "isa.h"
> +#include "pc.h"
> +
> +//#define DEBUG_DEBUGCON
> +
> +typedef struct DebugconState {
> +    CharDriverState *chr;
> +    uint32_t readback;
> +} DebugconState;
> +
> +typedef struct ISADebugconState {
> +    ISADevice dev;
> +    uint32_t iobase;
> +    DebugconState state;
> +} ISADebugconState;
> +
> +static void debugcon_ioport_write(void *opaque, uint32_t addr, uint32_t val)
> +{
> +    DebugconState *s = opaque;
> +    unsigned char ch = val;
> +
> +#ifdef DEBUG_DEBUGCON
> +    printf("debugcon: write addr=0x%04x val=0x%02x\n", addr, val);
> +#endif
> +
> +    qemu_chr_write(s->chr,&ch, 1);
> +}
> +
> +
> +static uint32_t debugcon_ioport_read(void *opaque, uint32_t addr)
> +{
> +    DebugconState *s = opaque;
> +
> +#ifdef DEBUG_DEBUGCON
> +    printf("debugcon: read addr=0x%04x\n", addr, val);
> +#endif
> +
> +    return s->readback;
> +}
> +
> +static void debugcon_init_core(DebugconState *s)
> +{
> +    if (!s->chr) {
> +        fprintf(stderr, "Can't create debugcon device, empty char device\n");
> +        exit(1);
> +    }
> +
> +    qemu_chr_add_handlers(s->chr, NULL, NULL, NULL, s);
> +}
> +
> +static int debugcon_isa_initfn(ISADevice *dev)
> +{
> +    ISADebugconState *isa = DO_UPCAST(ISADebugconState, dev, dev);
> +    DebugconState *s =&isa->state;
> +
> +    debugcon_init_core(s);
> +    register_ioport_write(isa->iobase, 1, 1, debugcon_ioport_write, s);
> +    register_ioport_read(isa->iobase, 1, 1, debugcon_ioport_read, s);
> +    return 0;
> +}
> +
> +static ISADeviceInfo debugcon_isa_info = {
> +    .qdev.name  = "isa-debugcon",
> +    .qdev.size  = sizeof(ISADebugconState),
> +    .init       = debugcon_isa_initfn,
> +    .qdev.props = (Property[]) {
> +        DEFINE_PROP_HEX32("iobase", ISADebugconState, iobase, 0xe9),
> +        DEFINE_PROP_CHR("chardev",  ISADebugconState, state.chr),
> +        DEFINE_PROP_HEX32("readback", ISADebugconState, state.readback, 0xe9),
> +        DEFINE_PROP_END_OF_LIST(),
> +    },
> +};
> +
> +static void debugcon_register_devices(void)
> +{
> +    isa_qdev_register(&debugcon_isa_info);
> +}
> +
> +device_init(debugcon_register_devices)
> diff --git a/qemu-options.hx b/qemu-options.hx
> index ecd50eb..9005fe8 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -1598,6 +1598,17 @@ STEXI
>   Setup monitor on chardev @var{name}.
>   ETEXI
>
> +DEF("debugcon", HAS_ARG, QEMU_OPTION_debugcon, \
> +    "-debugcon dev   redirect the debug console to char device 'dev'\n")
> +STEXI
> +@item -debugcon @var{dev}
> +Redirect the debug console to host device @var{dev} (same devices as the
> +serial port).  The debug console is an I/O port which is typically port
> +0xe9; writing to that I/O port sends output to this device.
> +The default device is @code{vc} in graphical mode and @code{stdio} in
> +non graphical mode.
> +ETEXI
> +
>   DEF("pidfile", HAS_ARG, QEMU_OPTION_pidfile, \
>       "-pidfile file   write PID to 'file'\n")
>   STEXI
> diff --git a/vl.c b/vl.c
> index e881e45..2b0b653 100644
> --- a/vl.c
> +++ b/vl.c
> @@ -4748,6 +4748,7 @@ struct device_config {
>           DEV_SERIAL,    /* -serial        */
>           DEV_PARALLEL,  /* -parallel      */
>           DEV_VIRTCON,   /* -virtioconsole */
> +        DEV_DEBUGCON,  /* -debugcon */
>       } type;
>       const char *cmdline;
>       QTAILQ_ENTRY(device_config) next;
> @@ -4845,6 +4846,23 @@ static int virtcon_parse(const char *devname)
>       return 0;
>   }
>
> +static int debugcon_parse(const char *devname)
> +{
> +    QemuOpts *opts;
> +
> +    if (!qemu_chr_open("debugcon", devname, NULL)) {
> +        exit(1);
> +    }
> +    opts = qemu_opts_create(&qemu_device_opts, "debugcon", 1);
> +    if (!opts) {
> +        fprintf(stderr, "qemu: already have a debugcon device\n");
> +        exit(1);
> +    }
> +    qemu_opt_set(opts, "driver", "isa-debugcon");
> +    qemu_opt_set(opts, "chardev", "debugcon");
> +    return 0;
> +}
> +
>   int main(int argc, char **argv, char **envp)
>   {
>       const char *gdbstub_dev = NULL;
> @@ -5390,6 +5408,9 @@ int main(int argc, char **argv, char **envp)
>                   add_device_config(DEV_PARALLEL, optarg);
>                   default_parallel = 0;
>                   break;
> +            case QEMU_OPTION_debugcon:
> +                add_device_config(DEV_DEBUGCON, optarg);
> +                break;
>   	    case QEMU_OPTION_loadvm:
>   		loadvm = optarg;
>   		break;
> @@ -5927,6 +5948,8 @@ int main(int argc, char **argv, char **envp)
>           exit(1);
>       if (foreach_device_config(DEV_VIRTCON, virtcon_parse)<  0)
>           exit(1);
> +    if (foreach_device_config(DEV_DEBUGCON, debugcon_parse)<  0)
> +        exit(1);
>
>       module_call_init(MODULE_INIT_DEVICE);
>
>

Patch

diff --git a/Makefile.target b/Makefile.target
index 7c1f30c..0a803ef 100644
--- a/Makefile.target
+++ b/Makefile.target
@@ -194,7 +194,7 @@  obj-i386-y += fdc.o mc146818rtc.o serial.o i8259.o i8254.o pcspk.o pc.o
 obj-i386-y += cirrus_vga.o apic.o ioapic.o parallel.o acpi.o piix_pci.o
 obj-i386-y += usb-uhci.o vmmouse.o vmport.o vmware_vga.o hpet.o
 obj-i386-y += device-hotplug.o pci-hotplug.o smbios.o wdt_ib700.o
-obj-i386-y += ne2000-isa.o
+obj-i386-y += ne2000-isa.o debugcon.o
 
 # shared objects
 obj-ppc-y = ppc.o ide/core.o ide/qdev.o ide/isa.o ide/pci.o ide/macio.o
diff --git a/hw/debugcon.c b/hw/debugcon.c
new file mode 100644
index 0000000..d549091
--- /dev/null
+++ b/hw/debugcon.c
@@ -0,0 +1,107 @@ 
+/*
+ * QEMU Bochs-style debug console ("port E9") emulation
+ *
+ * Copyright (c) 2003-2004 Fabrice Bellard
+ * Copyright (c) 2008 Citrix Systems, Inc.
+ * Copyright (c) Intel Corporation; author: H. Peter Anvin
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
+ * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+#include "hw.h"
+#include "qemu-char.h"
+#include "isa.h"
+#include "pc.h"
+
+//#define DEBUG_DEBUGCON
+
+typedef struct DebugconState {
+    CharDriverState *chr;
+    uint32_t readback;
+} DebugconState;
+
+typedef struct ISADebugconState {
+    ISADevice dev;
+    uint32_t iobase;
+    DebugconState state;
+} ISADebugconState;
+
+static void debugcon_ioport_write(void *opaque, uint32_t addr, uint32_t val)
+{
+    DebugconState *s = opaque;
+    unsigned char ch = val;
+
+#ifdef DEBUG_DEBUGCON
+    printf("debugcon: write addr=0x%04x val=0x%02x\n", addr, val);
+#endif
+
+    qemu_chr_write(s->chr, &ch, 1);
+}
+
+
+static uint32_t debugcon_ioport_read(void *opaque, uint32_t addr)
+{
+    DebugconState *s = opaque;
+
+#ifdef DEBUG_DEBUGCON
+    printf("debugcon: read addr=0x%04x\n", addr, val);
+#endif
+
+    return s->readback;
+}
+
+static void debugcon_init_core(DebugconState *s)
+{
+    if (!s->chr) {
+        fprintf(stderr, "Can't create debugcon device, empty char device\n");
+        exit(1);
+    }
+
+    qemu_chr_add_handlers(s->chr, NULL, NULL, NULL, s);
+}
+
+static int debugcon_isa_initfn(ISADevice *dev)
+{
+    ISADebugconState *isa = DO_UPCAST(ISADebugconState, dev, dev);
+    DebugconState *s = &isa->state;
+
+    debugcon_init_core(s);
+    register_ioport_write(isa->iobase, 1, 1, debugcon_ioport_write, s);
+    register_ioport_read(isa->iobase, 1, 1, debugcon_ioport_read, s);
+    return 0;
+}
+
+static ISADeviceInfo debugcon_isa_info = {
+    .qdev.name  = "isa-debugcon",
+    .qdev.size  = sizeof(ISADebugconState),
+    .init       = debugcon_isa_initfn,
+    .qdev.props = (Property[]) {
+        DEFINE_PROP_HEX32("iobase", ISADebugconState, iobase, 0xe9),
+        DEFINE_PROP_CHR("chardev",  ISADebugconState, state.chr),
+        DEFINE_PROP_HEX32("readback", ISADebugconState, state.readback, 0xe9),
+        DEFINE_PROP_END_OF_LIST(),
+    },
+};
+
+static void debugcon_register_devices(void)
+{
+    isa_qdev_register(&debugcon_isa_info);
+}
+
+device_init(debugcon_register_devices)
diff --git a/qemu-options.hx b/qemu-options.hx
index ecd50eb..9005fe8 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -1598,6 +1598,17 @@  STEXI
 Setup monitor on chardev @var{name}.
 ETEXI
 
+DEF("debugcon", HAS_ARG, QEMU_OPTION_debugcon, \
+    "-debugcon dev   redirect the debug console to char device 'dev'\n")
+STEXI
+@item -debugcon @var{dev}
+Redirect the debug console to host device @var{dev} (same devices as the
+serial port).  The debug console is an I/O port which is typically port
+0xe9; writing to that I/O port sends output to this device.
+The default device is @code{vc} in graphical mode and @code{stdio} in
+non graphical mode.
+ETEXI
+
 DEF("pidfile", HAS_ARG, QEMU_OPTION_pidfile, \
     "-pidfile file   write PID to 'file'\n")
 STEXI
diff --git a/vl.c b/vl.c
index e881e45..2b0b653 100644
--- a/vl.c
+++ b/vl.c
@@ -4748,6 +4748,7 @@  struct device_config {
         DEV_SERIAL,    /* -serial        */
         DEV_PARALLEL,  /* -parallel      */
         DEV_VIRTCON,   /* -virtioconsole */
+        DEV_DEBUGCON,  /* -debugcon */
     } type;
     const char *cmdline;
     QTAILQ_ENTRY(device_config) next;
@@ -4845,6 +4846,23 @@  static int virtcon_parse(const char *devname)
     return 0;
 }
 
+static int debugcon_parse(const char *devname)
+{   
+    QemuOpts *opts;
+
+    if (!qemu_chr_open("debugcon", devname, NULL)) {
+        exit(1);
+    }
+    opts = qemu_opts_create(&qemu_device_opts, "debugcon", 1);
+    if (!opts) {
+        fprintf(stderr, "qemu: already have a debugcon device\n");
+        exit(1);
+    }
+    qemu_opt_set(opts, "driver", "isa-debugcon");
+    qemu_opt_set(opts, "chardev", "debugcon");
+    return 0;
+}
+
 int main(int argc, char **argv, char **envp)
 {
     const char *gdbstub_dev = NULL;
@@ -5390,6 +5408,9 @@  int main(int argc, char **argv, char **envp)
                 add_device_config(DEV_PARALLEL, optarg);
                 default_parallel = 0;
                 break;
+            case QEMU_OPTION_debugcon:
+                add_device_config(DEV_DEBUGCON, optarg);
+                break;
 	    case QEMU_OPTION_loadvm:
 		loadvm = optarg;
 		break;
@@ -5927,6 +5948,8 @@  int main(int argc, char **argv, char **envp)
         exit(1);
     if (foreach_device_config(DEV_VIRTCON, virtcon_parse) < 0)
         exit(1);
+    if (foreach_device_config(DEV_DEBUGCON, debugcon_parse) < 0)
+        exit(1);
 
     module_call_init(MODULE_INIT_DEVICE);