[PATCH v3] xen/common: add keyhandler to show Xen command line

[PATCH v3] xen/common: add keyhandler to show Xen command line

[dmukhin]

From: Denis Mukhin <dmukhin@ford.com> 

Currently there's no way to print Xen command line on the emergency
console for debugging purposes when 'xl' is not unavailable.

Add new keyhander 'X' to do command line printout.

Signed-off-by: Denis Mukhin <dmukhin@ford.com>
---
Changes since v2:
- account for CONFIG_CMDLINE_OVERRIDE case
- use IS_ENABLED()

v2: https://lore.kernel.org/xen-devel/20260803070047.3097846-3-dmukhin@ford.com/
CI: https://gitlab.com/xen-project/people/dmukhin/xen/-/pipelines/2748655201
---
 xen/common/kernel.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/xen/common/kernel.c b/xen/common/kernel.c
index d1bef9ac2b2b..54a7ee6f68e5 100644
--- a/xen/common/kernel.c
+++ b/xen/common/kernel.c
@@ -5,6 +5,7 @@
  */
 
 #include <xen/init.h>
+#include <xen/keyhandler.h>
 #include <xen/lib.h>
 #include <xen/errno.h>
 #include <xen/param.h>
@@ -505,6 +506,25 @@ static int __init cf_check param_init(void)
 __initcall(param_init);
 #endif
 
+static void cf_check show_hypervisor_info(unsigned char key)
+{
+    printk("'%c' pressed -> showing hypervisor information\n", key);
+
+    if ( IS_ENABLED(CONFIG_CMDLINE_OVERRIDE) )
+        printk("Bootloader command line ignored (CONFIG_CMDLINE_OVERRIDE=y)\n");
+    else
+        printk("Command line: %s\n", saved_cmdline);
+}
+
+static int __init cf_check misc_init(void)
+{
+    register_keyhandler('X', show_hypervisor_info,
+                        "show hypervisor information", 0);
+
+    return 0;
+}
+__initcall(misc_init);
+
 static long xenver_varbuf_op(int cmd, XEN_GUEST_HANDLE_PARAM(void) arg)
 {
     struct xen_varbuf user_str;
-- 
2.54.0

Re: [PATCH v3] xen/common: add keyhandler to show Xen command line

[Roger Pau Monné]

On Mon, Aug 10, 2026 at 04:04:01PM -0700, dmukhin@ford.com wrote:
> From: Denis Mukhin <dmukhin@ford.com> 
> 
> Currently there's no way to print Xen command line on the emergency
> console for debugging purposes when 'xl' is not unavailable.
> 
> Add new keyhander 'X' to do command line printout.
> 
> Signed-off-by: Denis Mukhin <dmukhin@ford.com>
> ---
> Changes since v2:
> - account for CONFIG_CMDLINE_OVERRIDE case
> - use IS_ENABLED()
> 
> v2: https://lore.kernel.org/xen-devel/20260803070047.3097846-3-dmukhin@ford.com/
> CI: https://gitlab.com/xen-project/people/dmukhin/xen/-/pipelines/2748655201
> ---
>  xen/common/kernel.c | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
> 
> diff --git a/xen/common/kernel.c b/xen/common/kernel.c
> index d1bef9ac2b2b..54a7ee6f68e5 100644
> --- a/xen/common/kernel.c
> +++ b/xen/common/kernel.c
> @@ -5,6 +5,7 @@
>   */
>  
>  #include <xen/init.h>
> +#include <xen/keyhandler.h>
>  #include <xen/lib.h>
>  #include <xen/errno.h>
>  #include <xen/param.h>
> @@ -505,6 +506,25 @@ static int __init cf_check param_init(void)
>  __initcall(param_init);
>  #endif
>  
> +static void cf_check show_hypervisor_info(unsigned char key)
> +{
> +    printk("'%c' pressed -> showing hypervisor information\n", key);
> +
> +    if ( IS_ENABLED(CONFIG_CMDLINE_OVERRIDE) )
> +        printk("Bootloader command line ignored (CONFIG_CMDLINE_OVERRIDE=y)\n");

Since we are there already, why not print the builtin command line
using CONFIG_CMDLINE?

> +    else
> +        printk("Command line: %s\n", saved_cmdline);
> +}
> +
> +static int __init cf_check misc_init(void)
> +{
> +    register_keyhandler('X', show_hypervisor_info,
> +                        "show hypervisor information", 0);

"show hypervisor information" seems too generic to me, almost all
debug keys could be defined by this sentence TBH.  I think this needs
to be more specific, but I'm not sure what's the plan regarding this
key.  Is there an intention to print more stuff here, or just the
command line?  Knowing the full set of information to be printed might
help come up with a better name.

Thanks, Roger.

Re: [PATCH v3] xen/common: add keyhandler to show Xen command line

[dmukhin]

Thanks for taking a look!

On Tue, Aug 11, 2026 at 10:05:17AM +0200, Roger Pau Monné wrote:
> On Mon, Aug 10, 2026 at 04:04:01PM -0700, dmukhin@ford.com wrote:
> > From: Denis Mukhin <dmukhin@ford.com> 
> > 
> > Currently there's no way to print Xen command line on the emergency
> > console for debugging purposes when 'xl' is not unavailable.
> > 
> > Add new keyhander 'X' to do command line printout.
> > 
> > Signed-off-by: Denis Mukhin <dmukhin@ford.com>
> > ---
> > Changes since v2:
> > - account for CONFIG_CMDLINE_OVERRIDE case
> > - use IS_ENABLED()
> > 
> > v2: https://lore.kernel.org/xen-devel/20260803070047.3097846-3-dmukhin@ford.com/
> > CI: https://gitlab.com/xen-project/people/dmukhin/xen/-/pipelines/2748655201
> > ---
> >  xen/common/kernel.c | 20 ++++++++++++++++++++
> >  1 file changed, 20 insertions(+)
> > 
> > diff --git a/xen/common/kernel.c b/xen/common/kernel.c
> > index d1bef9ac2b2b..54a7ee6f68e5 100644
> > --- a/xen/common/kernel.c
> > +++ b/xen/common/kernel.c
> > @@ -5,6 +5,7 @@
> >   */
> >  
> >  #include <xen/init.h>
> > +#include <xen/keyhandler.h>
> >  #include <xen/lib.h>
> >  #include <xen/errno.h>
> >  #include <xen/param.h>
> > @@ -505,6 +506,25 @@ static int __init cf_check param_init(void)
> >  __initcall(param_init);
> >  #endif
> >  
> > +static void cf_check show_hypervisor_info(unsigned char key)
> > +{
> > +    printk("'%c' pressed -> showing hypervisor information\n", key);
> > +
> > +    if ( IS_ENABLED(CONFIG_CMDLINE_OVERRIDE) )
> > +        printk("Bootloader command line ignored (CONFIG_CMDLINE_OVERRIDE=y)\n");
> 
> Since we are there already, why not print the builtin command line
> using CONFIG_CMDLINE?

Will update.

> 
> > +    else
> > +        printk("Command line: %s\n", saved_cmdline);
> > +}
> > +
> > +static int __init cf_check misc_init(void)
> > +{
> > +    register_keyhandler('X', show_hypervisor_info,
> > +                        "show hypervisor information", 0);
> 
> "show hypervisor information" seems too generic to me, almost all
> debug keys could be defined by this sentence TBH.  I think this needs
> to be more specific, but I'm not sure what's the plan regarding this
> key.  Is there an intention to print more stuff here, or just the
> command line?  Knowing the full set of information to be printed might
> help come up with a better name.

I will update to "show_cmdline" since I originally planed to expose Xen
command line only so it is possible to better debug a system when dom0
becomes almost unresponsive.

> 
> Thanks, Roger.
> 

--
Denis

Re: [PATCH v3] xen/common: add keyhandler to show Xen command line

[Jan Beulich]

On 11.08.2026 22:56, dmukhin@ford.com wrote:
> On Tue, Aug 11, 2026 at 10:05:17AM +0200, Roger Pau Monné wrote:
>> On Mon, Aug 10, 2026 at 04:04:01PM -0700, dmukhin@ford.com wrote:
>>> +static int __init cf_check misc_init(void)
>>> +{
>>> +    register_keyhandler('X', show_hypervisor_info,
>>> +                        "show hypervisor information", 0);
>>
>> "show hypervisor information" seems too generic to me, almost all
>> debug keys could be defined by this sentence TBH.  I think this needs
>> to be more specific, but I'm not sure what's the plan regarding this
>> key.  Is there an intention to print more stuff here, or just the
>> command line?  Knowing the full set of information to be printed might
>> help come up with a better name.
> 
> I will update to "show_cmdline" since I originally planed to expose Xen
> command line only so it is possible to better debug a system when dom0
> becomes almost unresponsive.

Yet as indicated already on v1 (I think) - a precious debug key character
for just the command line seems rather wasteful to me. If it's only the
command line, and if that _really_ needs exposing via a debug key (i.e.
if there are reasonable scenarios where "xl info" cannot be used), perhaps
attach it to e.g. the 'h' key output?

Jan

Re: [PATCH v3] xen/common: add keyhandler to show Xen command line

[Roger Pau Monné]

On Wed, Aug 12, 2026 at 09:28:46AM +0200, Jan Beulich wrote:
> On 11.08.2026 22:56, dmukhin@ford.com wrote:
> > On Tue, Aug 11, 2026 at 10:05:17AM +0200, Roger Pau Monné wrote:
> >> On Mon, Aug 10, 2026 at 04:04:01PM -0700, dmukhin@ford.com wrote:
> >>> +static int __init cf_check misc_init(void)
> >>> +{
> >>> +    register_keyhandler('X', show_hypervisor_info,
> >>> +                        "show hypervisor information", 0);
> >>
> >> "show hypervisor information" seems too generic to me, almost all
> >> debug keys could be defined by this sentence TBH.  I think this needs
> >> to be more specific, but I'm not sure what's the plan regarding this
> >> key.  Is there an intention to print more stuff here, or just the
> >> command line?  Knowing the full set of information to be printed might
> >> help come up with a better name.
> > 
> > I will update to "show_cmdline" since I originally planed to expose Xen
> > command line only so it is possible to better debug a system when dom0
> > becomes almost unresponsive.
> 
> Yet as indicated already on v1 (I think) - a precious debug key character
> for just the command line seems rather wasteful to me. If it's only the
> command line, and if that _really_ needs exposing via a debug key (i.e.
> if there are reasonable scenarios where "xl info" cannot be used), perhaps
> attach it to e.g. the 'h' key output?

I was going to say that we should not overload the 'h' key with
printing a possibly long string, but I see we already print a bunch of
information there, like the buildid and the compiler banner.

One option would be to introduce a new debug character, and move the
printing of the buildid and the banner to that key, together with the
command line.  Then 'h' output will be cleaner and just print the list
of installed handlers.  That would give the new key more content, and
help cleanup the output from the 'h' debug key at the same time.

Thanks, Roger.

Re: [PATCH v3] xen/common: add keyhandler to show Xen command line

[Jan Beulich]

On 12.08.2026 10:03, Roger Pau Monné wrote:
> On Wed, Aug 12, 2026 at 09:28:46AM +0200, Jan Beulich wrote:
>> On 11.08.2026 22:56, dmukhin@ford.com wrote:
>>> On Tue, Aug 11, 2026 at 10:05:17AM +0200, Roger Pau Monné wrote:
>>>> On Mon, Aug 10, 2026 at 04:04:01PM -0700, dmukhin@ford.com wrote:
>>>>> +static int __init cf_check misc_init(void)
>>>>> +{
>>>>> +    register_keyhandler('X', show_hypervisor_info,
>>>>> +                        "show hypervisor information", 0);
>>>>
>>>> "show hypervisor information" seems too generic to me, almost all
>>>> debug keys could be defined by this sentence TBH.  I think this needs
>>>> to be more specific, but I'm not sure what's the plan regarding this
>>>> key.  Is there an intention to print more stuff here, or just the
>>>> command line?  Knowing the full set of information to be printed might
>>>> help come up with a better name.
>>>
>>> I will update to "show_cmdline" since I originally planed to expose Xen
>>> command line only so it is possible to better debug a system when dom0
>>> becomes almost unresponsive.
>>
>> Yet as indicated already on v1 (I think) - a precious debug key character
>> for just the command line seems rather wasteful to me. If it's only the
>> command line, and if that _really_ needs exposing via a debug key (i.e.
>> if there are reasonable scenarios where "xl info" cannot be used), perhaps
>> attach it to e.g. the 'h' key output?
> 
> I was going to say that we should not overload the 'h' key with
> printing a possibly long string, but I see we already print a bunch of
> information there, like the buildid and the compiler banner.
> 
> One option would be to introduce a new debug character, and move the
> printing of the buildid and the banner to that key, together with the
> command line.  Then 'h' output will be cleaner and just print the list
> of installed handlers.  That would give the new key more content, and
> help cleanup the output from the 'h' debug key at the same time.

Hmm, yes, that's definitely an option. May I then further suggest to use
'?' as the key for this? (Or am I overlooking that key already being in
use somewhere?)

Jan