[PATCH V2 0/3] Emergency write support for virtio console devices.

[PATCH V2 0/3] Emergency write support for virtio console devices.

[Pranavkumar Sawargaonkar]

This patch-set implements early printk support for virtio console
devices by adding and using emergency write register to virtio console's 
config space.

The current virtio early printk code in kernel expects that hypervisor 
will provide some mechanism generally a hypercall to support early printk. 
This patch-set does not break existing hypercall based early print support.

This implementation adds:
1. Emergency writeonly register named emerg_wr in 
   virtio console's config space.
2. Host feature flags namely VIRTIO_CONSOLE_F_EMERG_WRITE
   for telling guest about early-write capability in console device.

Emergency write mechanism:
1. When a guest wants to out some character, it has to simply write 
   the character to emerg_wr register in config space of 
   virtio console device.

Pranavkumar Sawargaonkar (3):
  virtio: console: Add emergency writeonly register to config space
  Documentation: virtio: Add emergency write (emerg_wr) config register
        in virtio console.
  arm64: earlyprintk support for virtio-mmio console.

 Documentation/virtual/virtio-spec.txt |    8 +++++++-
 arch/arm64/kernel/early_printk.c      |   35 +++++++++++++++++++++++++++++++++
 include/uapi/linux/virtio_console.h   |    3 +++
 3 files changed, 45 insertions(+), 1 deletion(-)

-- 
1.7.9.5

[PATCH V2 1/3] virtio: console: Add emergency writeonly register to config space

[Pranavkumar Sawargaonkar]

This patch adds an emerg_wr register (writeonly) in config space
of virtio console device which can be used for debugging.

Signed-off-by: Pranavkumar Sawargaonkar <pranavkumar@linaro.org>
Signed-off-by: Anup Patel <anup.patel@linaro.org>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
---
 include/uapi/linux/virtio_console.h |    3 +++
 1 file changed, 3 insertions(+)

diff --git a/include/uapi/linux/virtio_console.h b/include/uapi/linux/virtio_console.h
index ee13ab6..3fd0dce 100644
--- a/include/uapi/linux/virtio_console.h
+++ b/include/uapi/linux/virtio_console.h
@@ -38,6 +38,7 @@
 /* Feature bits */
 #define VIRTIO_CONSOLE_F_SIZE	0	/* Does host provide console size? */
 #define VIRTIO_CONSOLE_F_MULTIPORT 1	/* Does host provide multiple ports? */
+#define VIRTIO_CONSOLE_F_EMERG_WRITE 2 /* Does host support emergency write? */
 
 #define VIRTIO_CONSOLE_BAD_ID		(~(u32)0)
 
@@ -48,6 +49,8 @@ struct virtio_console_config {
 	__u16 rows;
 	/* max. number of ports this device can hold */
 	__u32 max_nr_ports;
+	/* emergency write register */
+	__u32 emerg_wr;
 } __attribute__((packed));
 
 /*
-- 
1.7.9.5

[PATCH V2 1/3] virtio: console: Add emergency writeonly register to config space

[Amit Shah]

On (Mon) 06 May 2013 [17:49:49], Pranavkumar Sawargaonkar wrote:
> This patch adds an emerg_wr register (writeonly) in config space
> of virtio console device which can be used for debugging.
> 
> Signed-off-by: Pranavkumar Sawargaonkar <pranavkumar@linaro.org>
> Signed-off-by: Anup Patel <anup.patel@linaro.org>
> Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

OK, this change by itself looks harmless.

Acked-by: Amit Shah <amit.shah@redhat.com>

		Amit

[PATCH V2 1/3] virtio: console: Add emergency writeonly register to config space

[Rusty Russell]

Amit Shah <amit.shah@redhat.com> writes:
> On (Mon) 06 May 2013 [17:49:49], Pranavkumar Sawargaonkar wrote:
>> This patch adds an emerg_wr register (writeonly) in config space
>> of virtio console device which can be used for debugging.
>> 
>> Signed-off-by: Pranavkumar Sawargaonkar <pranavkumar@linaro.org>
>> Signed-off-by: Anup Patel <anup.patel@linaro.org>
>> Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
>
> OK, this change by itself looks harmless.
>
> Acked-by: Amit Shah <amit.shah@redhat.com>
>
> 		Amit

Thanks.  This is queued for *next* merge window.

Cheers,
Rusty.

[PATCH V2 1/3] virtio: console: Add emergency writeonly register to config space

[Pranavkumar Sawargaonkar]

On 17 May 2013 05:22, Rusty Russell <rusty@rustcorp.com.au> wrote:
> Amit Shah <amit.shah@redhat.com> writes:
>> On (Mon) 06 May 2013 [17:49:49], Pranavkumar Sawargaonkar wrote:
>>> This patch adds an emerg_wr register (writeonly) in config space
>>> of virtio console device which can be used for debugging.
>>>
>>> Signed-off-by: Pranavkumar Sawargaonkar <pranavkumar@linaro.org>
>>> Signed-off-by: Anup Patel <anup.patel@linaro.org>
>>> Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
>>
>> OK, this change by itself looks harmless.
>>
>> Acked-by: Amit Shah <amit.shah@redhat.com>
>>
>>               Amit
>
> Thanks.  This is queued for *next* merge window.
>

Thanks amit and rusty.

> Cheers,
> Rusty.

Regards,
Pranav

[PATCH V2 2/3] Documentation: virtio: Add emergency write (emerg_wr) config register in virtio console.

[Pranavkumar Sawargaonkar]

Signed-off-by: Pranavkumar Sawargaonkar <pranavkumar@linaro.org>
Signed-off-by: Anup Patel <anup.patel@linaro.org>
---
 Documentation/virtual/virtio-spec.txt |    8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/Documentation/virtual/virtio-spec.txt b/Documentation/virtual/virtio-spec.txt
index 0d6ec85..3ca38f8 100644
--- a/Documentation/virtual/virtio-spec.txt
+++ b/Documentation/virtual/virtio-spec.txt
@@ -1927,11 +1927,16 @@ Ports 2 onwards only if VIRTIO_CONSOLE_F_MULTIPORT is set
     ports; configuration fields nr_ports and max_nr_ports are
     valid and control virtqueues will be used.
 
+  VIRTIO_CONSOLE_F_EMERG_WRITE(2) Device has support for emergency
+  write. Configuration filed emerg_wr is valid.
+
   Device configuration layout The size of the console is supplied
   in the configuration space if the VIRTIO_CONSOLE_F_SIZE feature
   is set. Furthermore, if the VIRTIO_CONSOLE_F_MULTIPORT feature
   is set, the maximum number of ports supported by the device can
-  be fetched.struct virtio_console_config {
+  be fetched. If VIRTIO_CONSOLE_F_EMERG_WRITE is set then one can
+  use emergency write to output single character without initializing
+  virtio queues. struct virtio_console_config {
 
 	u16 cols;
 
@@ -1941,6 +1946,7 @@ Ports 2 onwards only if VIRTIO_CONSOLE_F_MULTIPORT is set
 
 	u32 max_nr_ports;
 
+	u32 emerg_wr;
 };
 
   Device Initialization
-- 
1.7.9.5

[PATCH V2 2/3] Documentation: virtio: Add emergency write (emerg_wr) config register in virtio console.

[Rob Landley]

On 05/06/2013 07:19:50 AM, Pranavkumar Sawargaonkar wrote:
> Signed-off-by: Pranavkumar Sawargaonkar <pranavkumar@linaro.org>
> Signed-off-by: Anup Patel <anup.patel@linaro.org>
> ---
>  Documentation/virtual/virtio-spec.txt |    8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/virtual/virtio-spec.txt  
> b/Documentation/virtual/virtio-spec.txt
> index 0d6ec85..3ca38f8 100644
> --- a/Documentation/virtual/virtio-spec.txt
> +++ b/Documentation/virtual/virtio-spec.txt
> @@ -1927,11 +1927,16 @@ Ports 2 onwards only if  
> VIRTIO_CONSOLE_F_MULTIPORT is set
>      ports; configuration fields nr_ports and max_nr_ports are
>      valid and control virtqueues will be used.
> 
> +  VIRTIO_CONSOLE_F_EMERG_WRITE(2) Device has support for emergency
> +  write. Configuration filed emerg_wr is valid.

Emergency?

Really?

Out of morbid curiosity, what would constitute an _emergency_ in a  
virtual machine logging context?

Rob

[PATCH V2 2/3] Documentation: virtio: Add emergency write (emerg_wr) config register in virtio console.

[Rusty Russell]

Rob Landley <rob@landley.net> writes:
> On 05/06/2013 07:19:50 AM, Pranavkumar Sawargaonkar wrote:
>> Signed-off-by: Pranavkumar Sawargaonkar <pranavkumar@linaro.org>
>> Signed-off-by: Anup Patel <anup.patel@linaro.org>
>> ---
>>  Documentation/virtual/virtio-spec.txt |    8 +++++++-
>>  1 file changed, 7 insertions(+), 1 deletion(-)
>> 
>> diff --git a/Documentation/virtual/virtio-spec.txt  
>> b/Documentation/virtual/virtio-spec.txt
>> index 0d6ec85..3ca38f8 100644
>> --- a/Documentation/virtual/virtio-spec.txt
>> +++ b/Documentation/virtual/virtio-spec.txt
>> @@ -1927,11 +1927,16 @@ Ports 2 onwards only if  
>> VIRTIO_CONSOLE_F_MULTIPORT is set
>>      ports; configuration fields nr_ports and max_nr_ports are
>>      valid and control virtqueues will be used.
>> 
>> +  VIRTIO_CONSOLE_F_EMERG_WRITE(2) Device has support for emergency
>> +  write. Configuration filed emerg_wr is valid.
>
> Emergency?
>
> Really?
>
> Out of morbid curiosity, what would constitute an _emergency_ in a  
> virtual machine logging context?

Usually, your console device is broken.  Or you can't set it up because
you can't allocate memory.  Or you oops before it is configured.

Terminology is a tricky thing, but the nuance here is that you don't
want to use this as your main console.  It may not do anything.  It may
drop characters.  It's definitely slow.

Cheers,
Rusty.

[PATCH V2 2/3] Documentation: virtio: Add emergency write (emerg_wr) config register in virtio console.

[Rusty Russell]

Pranavkumar Sawargaonkar <pranavkumar@linaro.org> writes:
> Signed-off-by: Pranavkumar Sawargaonkar <pranavkumar@linaro.org>
> Signed-off-by: Anup Patel <anup.patel@linaro.org>
> ---
>  Documentation/virtual/virtio-spec.txt |    8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)

OK, I applied this to the lyx master, with three changes:
1) Changed "filed" to "field".
2) Added ", or even acknowledging the feature" after "without
   initializing virtio queues".
3) Added an initial point to the Device Initialization section:

   1. If the VIRTIO_CONSOLE_F_EMERG_WRITE feature is offered, the
   emerg_wr field of the configuration can be written at any time. Thus
   it should work for very early boot debugging output as well as
   catastophic OS failures (eg. virtio ring corruption).

Thanks,
Rusty.

> diff --git a/Documentation/virtual/virtio-spec.txt b/Documentation/virtual/virtio-spec.txt
> index 0d6ec85..3ca38f8 100644
> --- a/Documentation/virtual/virtio-spec.txt
> +++ b/Documentation/virtual/virtio-spec.txt
> @@ -1927,11 +1927,16 @@ Ports 2 onwards only if VIRTIO_CONSOLE_F_MULTIPORT is set
>      ports; configuration fields nr_ports and max_nr_ports are
>      valid and control virtqueues will be used.
>  
> +  VIRTIO_CONSOLE_F_EMERG_WRITE(2) Device has support for emergency
> +  write. Configuration filed emerg_wr is valid.
> +
>    Device configuration layout The size of the console is supplied
>    in the configuration space if the VIRTIO_CONSOLE_F_SIZE feature
>    is set. Furthermore, if the VIRTIO_CONSOLE_F_MULTIPORT feature
>    is set, the maximum number of ports supported by the device can
> -  be fetched.struct virtio_console_config {
> +  be fetched. If VIRTIO_CONSOLE_F_EMERG_WRITE is set then one can
> +  use emergency write to output single character without initializing
> +  virtio queues. struct virtio_console_config {
>  
>  	u16 cols;
>  
> @@ -1941,6 +1946,7 @@ Ports 2 onwards only if VIRTIO_CONSOLE_F_MULTIPORT is set
>  
>  	u32 max_nr_ports;
>  
> +	u32 emerg_wr;
>  };
>  
>    Device Initialization
> -- 
> 1.7.9.5

[PATCH V2 2/3] Documentation: virtio: Add emergency write (emerg_wr) config register in virtio console.

[Pranavkumar Sawargaonkar]

Hi Rusty,

On 13 May 2013 08:22, Rusty Russell <rusty@rustcorp.com.au> wrote:
> Pranavkumar Sawargaonkar <pranavkumar@linaro.org> writes:
>> Signed-off-by: Pranavkumar Sawargaonkar <pranavkumar@linaro.org>
>> Signed-off-by: Anup Patel <anup.patel@linaro.org>
>> ---
>>  Documentation/virtual/virtio-spec.txt |    8 +++++++-
>>  1 file changed, 7 insertions(+), 1 deletion(-)
>
> OK, I applied this to the lyx master, with three changes:
> 1) Changed "filed" to "field".
> 2) Added ", or even acknowledging the feature" after "without
>    initializing virtio queues".
> 3) Added an initial point to the Device Initialization section:
>
>    1. If the VIRTIO_CONSOLE_F_EMERG_WRITE feature is offered, the
>    emerg_wr field of the configuration can be written at any time. Thus
>    it should work for very early boot debugging output as well as
>    catastophic OS failures (eg. virtio ring corruption).

Thanks for applying this patch.
Have you also applied first patch ([PATCH V2 1/3] virtio: console: Add
emergency writeonly register to config space)
with this?  (https://lkml.org/lkml/2013/5/6/169)

>
> Thanks,
> Rusty.

Thanks,
Pranav

>
>> diff --git a/Documentation/virtual/virtio-spec.txt b/Documentation/virtual/virtio-spec.txt
>> index 0d6ec85..3ca38f8 100644
>> --- a/Documentation/virtual/virtio-spec.txt
>> +++ b/Documentation/virtual/virtio-spec.txt
>> @@ -1927,11 +1927,16 @@ Ports 2 onwards only if VIRTIO_CONSOLE_F_MULTIPORT is set
>>      ports; configuration fields nr_ports and max_nr_ports are
>>      valid and control virtqueues will be used.
>>
>> +  VIRTIO_CONSOLE_F_EMERG_WRITE(2) Device has support for emergency
>> +  write. Configuration filed emerg_wr is valid.
>> +
>>    Device configuration layout The size of the console is supplied
>>    in the configuration space if the VIRTIO_CONSOLE_F_SIZE feature
>>    is set. Furthermore, if the VIRTIO_CONSOLE_F_MULTIPORT feature
>>    is set, the maximum number of ports supported by the device can
>> -  be fetched.struct virtio_console_config {
>> +  be fetched. If VIRTIO_CONSOLE_F_EMERG_WRITE is set then one can
>> +  use emergency write to output single character without initializing
>> +  virtio queues. struct virtio_console_config {
>>
>>       u16 cols;
>>
>> @@ -1941,6 +1946,7 @@ Ports 2 onwards only if VIRTIO_CONSOLE_F_MULTIPORT is set
>>
>>       u32 max_nr_ports;
>>
>> +     u32 emerg_wr;
>>  };
>>
>>    Device Initialization
>> --
>> 1.7.9.5

[PATCH V2 2/3] Documentation: virtio: Add emergency write (emerg_wr) config register in virtio console.

[Christopher Covington]

Hi Pranavkumar,

On 05/13/2013 02:56 AM, Pranavkumar Sawargaonkar wrote:
> Hi Rusty,
> 
> On 13 May 2013 08:22, Rusty Russell <rusty@rustcorp.com.au> wrote:
>> Pranavkumar Sawargaonkar <pranavkumar@linaro.org> writes:
>>> Signed-off-by: Pranavkumar Sawargaonkar <pranavkumar@linaro.org>
>>> Signed-off-by: Anup Patel <anup.patel@linaro.org>
>>> ---
>>>  Documentation/virtual/virtio-spec.txt |    8 +++++++-
>>>  1 file changed, 7 insertions(+), 1 deletion(-)
>>
>> OK, I applied this to the lyx master, with three changes:
>> 1) Changed "filed" to "field".
>> 2) Added ", or even acknowledging the feature" after "without
>>    initializing virtio queues".
>> 3) Added an initial point to the Device Initialization section:
>>
>>    1. If the VIRTIO_CONSOLE_F_EMERG_WRITE feature is offered, the
>>    emerg_wr field of the configuration can be written at any time. Thus
>>    it should work for very early boot debugging output as well as
>>    catastophic OS failures (eg. virtio ring corruption).
> 
> Thanks for applying this patch.
> Have you also applied first patch ([PATCH V2 1/3] virtio: console: Add
> emergency writeonly register to config space)
> with this?  (https://lkml.org/lkml/2013/5/6/169)

You seem to have omitted copying virtualization at lists.linux-foundation.org,
the mailing list documented in the MAINTAINERS file as appropriate for changes
relating to the virtio console driver.

[...]

Christopher

-- 
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
hosted by the Linux Foundation.

[PATCH V2 2/3] Documentation: virtio: Add emergency write (emerg_wr) config register in virtio console.

[Rusty Russell]

Pranavkumar Sawargaonkar <pranavkumar@linaro.org> writes:
> Hi Rusty,
>
> On 13 May 2013 08:22, Rusty Russell <rusty@rustcorp.com.au> wrote:
>> Pranavkumar Sawargaonkar <pranavkumar@linaro.org> writes:
>>> Signed-off-by: Pranavkumar Sawargaonkar <pranavkumar@linaro.org>
>>> Signed-off-by: Anup Patel <anup.patel@linaro.org>
>>> ---
>>>  Documentation/virtual/virtio-spec.txt |    8 +++++++-
>>>  1 file changed, 7 insertions(+), 1 deletion(-)
>>
>> OK, I applied this to the lyx master, with three changes:
>> 1) Changed "filed" to "field".
>> 2) Added ", or even acknowledging the feature" after "without
>>    initializing virtio queues".
>> 3) Added an initial point to the Device Initialization section:
>>
>>    1. If the VIRTIO_CONSOLE_F_EMERG_WRITE feature is offered, the
>>    emerg_wr field of the configuration can be written at any time. Thus
>>    it should work for very early boot debugging output as well as
>>    catastophic OS failures (eg. virtio ring corruption).
>
> Thanks for applying this patch.
> Have you also applied first patch ([PATCH V2 1/3] virtio: console: Add
> emergency writeonly register to config space)
> with this?  (https://lkml.org/lkml/2013/5/6/169)

Would like Amit's Ack, since he's de-facto console maintainer.

I've reproduced it below.

Cheers,
Rusty.

Subject: [PATCH V2 1/3] virtio: console: Add emergency writeonly register to config space
Date: Mon,  6 May 2013 17:49:49 +0530
Message-Id: <1367842791-30285-2-git-send-email-pranavkumar@linaro.org>
X-Mailer: git-send-email 1.7.9.5
In-Reply-To: <1367842791-30285-1-git-send-email-pranavkumar@linaro.org>
References: <1367842791-30285-1-git-send-email-pranavkumar@linaro.org>
X-Gm-Message-State: ALoCoQmDOoCdPmDpT+VgLwYAlJHmkUTB8sEAHVSbrNxWBzYgPHFT9rQ2A9dR8fToM81cHDZDppGx

This patch adds an emerg_wr register (writeonly) in config space
of virtio console device which can be used for debugging.

Signed-off-by: Pranavkumar Sawargaonkar <pranavkumar@linaro.org>
Signed-off-by: Anup Patel <anup.patel@linaro.org>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
---
 include/uapi/linux/virtio_console.h |    3 +++
 1 file changed, 3 insertions(+)

diff --git a/include/uapi/linux/virtio_console.h b/include/uapi/linux/virtio_console.h
index ee13ab6..3fd0dce 100644
--- a/include/uapi/linux/virtio_console.h
+++ b/include/uapi/linux/virtio_console.h
@@ -38,6 +38,7 @@
 /* Feature bits */
 #define VIRTIO_CONSOLE_F_SIZE	0	/* Does host provide console size? */
 #define VIRTIO_CONSOLE_F_MULTIPORT 1	/* Does host provide multiple ports? */
+#define VIRTIO_CONSOLE_F_EMERG_WRITE 2 /* Does host support emergency write? */
 
 #define VIRTIO_CONSOLE_BAD_ID		(~(u32)0)
 
@@ -48,6 +49,8 @@ struct virtio_console_config {
 	__u16 rows;
 	/* max. number of ports this device can hold */
 	__u32 max_nr_ports;
+	/* emergency write register */
+	__u32 emerg_wr;
 } __attribute__((packed));
 
 /*

[PATCH V2 2/3] Documentation: virtio: Add emergency write (emerg_wr) config register in virtio console.

[Amit Shah]

On (Wed) 15 May 2013 [13:59:29], Rusty Russell wrote:
> Pranavkumar Sawargaonkar <pranavkumar@linaro.org> writes:
> > Hi Rusty,
> >
> > On 13 May 2013 08:22, Rusty Russell <rusty@rustcorp.com.au> wrote:
> >> Pranavkumar Sawargaonkar <pranavkumar@linaro.org> writes:
> >>> Signed-off-by: Pranavkumar Sawargaonkar <pranavkumar@linaro.org>
> >>> Signed-off-by: Anup Patel <anup.patel@linaro.org>
> >>> ---
> >>>  Documentation/virtual/virtio-spec.txt |    8 +++++++-
> >>>  1 file changed, 7 insertions(+), 1 deletion(-)
> >>
> >> OK, I applied this to the lyx master, with three changes:
> >> 1) Changed "filed" to "field".
> >> 2) Added ", or even acknowledging the feature" after "without
> >>    initializing virtio queues".
> >> 3) Added an initial point to the Device Initialization section:
> >>
> >>    1. If the VIRTIO_CONSOLE_F_EMERG_WRITE feature is offered, the
> >>    emerg_wr field of the configuration can be written at any time. Thus
> >>    it should work for very early boot debugging output as well as
> >>    catastophic OS failures (eg. virtio ring corruption).
> >
> > Thanks for applying this patch.
> > Have you also applied first patch ([PATCH V2 1/3] virtio: console: Add
> > emergency writeonly register to config space)
> > with this?  (https://lkml.org/lkml/2013/5/6/169)
> 
> Would like Amit's Ack, since he's de-facto console maintainer.

I've been away, and am reviewing the discussions here.  Expect a
response later today.

Thanks,

		Amit

[PATCH V2 3/3] arm64: earlyprintk support for virtio-mmio console.

[Pranavkumar Sawargaonkar]

This patch implements earlyprintk based on virtio console
using emerg_wr config register.

Kernel args for using this will be:
earlyprintk=virtio-console,<phys_address>

Signed-off-by: Pranavkumar Sawargaonkar <pranavkumar@linaro.org>
Signed-off-by: Anup Patel <anup.patel@linaro.org>
---
 arch/arm64/kernel/early_printk.c |   35 +++++++++++++++++++++++++++++++++++
 1 file changed, 35 insertions(+)

diff --git a/arch/arm64/kernel/early_printk.c b/arch/arm64/kernel/early_printk.c
index ac974f4..ea3d2f5 100644
--- a/arch/arm64/kernel/early_printk.c
+++ b/arch/arm64/kernel/early_printk.c
@@ -25,6 +25,9 @@
 
 #include <linux/amba/serial.h>
 #include <linux/serial_reg.h>
+#include <linux/virtio_ids.h>
+#include <linux/virtio_mmio.h>
+#include <linux/virtio_console.h>
 
 static void __iomem *early_base;
 static void (*printch)(char ch);
@@ -53,6 +56,37 @@ static void smh_printch(char ch)
 }
 
 /*
+ * VIRTIO MMIO console single character Tx.
+ */
+static void virtio_console_printch(char ch)
+{
+	u32 tmp;
+	static u32 init_done;
+	static u32 can_write;
+	struct virtio_console_config *p = early_base + VIRTIO_MMIO_CONFIG;
+
+	if (!init_done) {
+		tmp = readl_relaxed(early_base + VIRTIO_MMIO_DEVICE_ID);
+		if (tmp != VIRTIO_ID_CONSOLE) {
+			init_done = 1;
+			return;
+		}
+
+		tmp = readl_relaxed(early_base + VIRTIO_MMIO_HOST_FEATURES);
+		if (!(tmp & (1 << VIRTIO_CONSOLE_F_EMERG_WRITE))) {
+			init_done = 1;
+			return;
+		}
+
+		init_done = 1;
+		can_write = 1;
+	}
+
+	if (can_write)
+		writeb_relaxed(ch, &p->emerg_wr);
+}
+
+/*
  * 8250/16550 (8-bit aligned registers) single character TX.
  */
 static void uart8250_8bit_printch(char ch)
@@ -82,6 +116,7 @@ static const struct earlycon_match earlycon_match[] __initconst = {
 	{ .name = "smh", .printch = smh_printch, },
 	{ .name = "uart8250-8bit", .printch = uart8250_8bit_printch, },
 	{ .name = "uart8250-32bit", .printch = uart8250_32bit_printch, },
+	{ .name = "virtio-console", .printch = virtio_console_printch, },
 	{}
 };
 
-- 
1.7.9.5