[PATCH v2 0/2] keyhandler to show Xen command line

[PATCH v2 0/2] keyhandler to show Xen command line

[dmukhin]

Tiny series which adds new 'X' keyhandler to show Xen command
line to help debugging the system.

CI: https://gitlab.com/xen-project/people/dmukhin/xen/-/pipelines/2723312941

Denis Mukhin (2):
  xen/common: annotate saved_cmdline with __ro_after_init
  xen/common: add keyhandler to show Xen command line

 xen/common/kernel.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

-- 
2.54.0

[PATCH v2 1/2] xen/common: annotate saved_cmdline with __ro_after_init

[dmukhin]

From: Denis Mukhin <dmukhin@ford.com> 

The hypervisor command line is not modified after initialization.
Annotate saved_cmdline with __ro_after_init to place it in the
dedicated read-only-after-init section.

Suggested-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Denis Mukhin <dmukhin@ford.com>
---
Changes since v1:
- new patch
---
 xen/common/kernel.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/xen/common/kernel.c b/xen/common/kernel.c
index fb45f8139995..d1bef9ac2b2b 100644
--- a/xen/common/kernel.c
+++ b/xen/common/kernel.c
@@ -34,7 +34,7 @@ bool __ro_after_init opt_dit = IS_ENABLED(CONFIG_DIT_DEFAULT);
 boolean_param("dit", opt_dit);
 #endif
 
-static xen_commandline_t saved_cmdline;
+static xen_commandline_t __ro_after_init saved_cmdline;
 static const char __initconst opt_builtin_cmdline[] = CONFIG_CMDLINE;
 char __ro_after_init xen_cap_info[128];
 
-- 
2.54.0

Re: [PATCH v2 1/2] xen/common: annotate saved_cmdline with __ro_after_init

[Jan Beulich]

On 03.08.2026 09:00, dmukhin@ford.com wrote:
> From: Denis Mukhin <dmukhin@ford.com> 
> 
> The hypervisor command line is not modified after initialization.
> Annotate saved_cmdline with __ro_after_init to place it in the
> dedicated read-only-after-init section.
> 
> Suggested-by: Jan Beulich <jbeulich@suse.com>
> Signed-off-by: Denis Mukhin <dmukhin@ford.com>

Acked-by: Jan Beulich <jbeulich@suse.com>

[PATCH v2 2/2] 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 (e.g. 'xl' is not available in dom0).

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

Signed-off-by: Denis Mukhin <dmukhin@ford.com>
---
v1: https://lore.kernel.org/xen-devel/20260730061459.2702672-2-dmukhin@ford.com/

Changes since v1:
- moved implementation into kernel.c
---
 xen/common/kernel.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/xen/common/kernel.c b/xen/common/kernel.c
index d1bef9ac2b2b..9f334c92e3ab 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 +506,21 @@ 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);
+    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 v2 2/2] xen/common: add keyhandler to show Xen command line

[Stefano Stabellini]

On Mon, 3 Aug 2026, 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 (e.g. 'xl' is not available in dom0).
> 
> Add new keyhander 'X' to do command line printout.
> 
> Signed-off-by: Denis Mukhin <dmukhin@ford.com>
> ---
> v1: https://lore.kernel.org/xen-devel/20260730061459.2702672-2-dmukhin@ford.com/
> 
> Changes since v1:
> - moved implementation into kernel.c
> ---
>  xen/common/kernel.c | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/xen/common/kernel.c b/xen/common/kernel.c
> index d1bef9ac2b2b..9f334c92e3ab 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 +506,21 @@ 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);
> +    printk("Command line: %s\n", saved_cmdline);

if CONFIG_CMDLINE_OVERRIDE is defined, saved_cmdline is empty. We could
at least do this:

#ifdef CONFIG_CMDLINE_OVERRIDE
    printk("Bootloader command line ignored (CONFIG_CMDLINE_OVERRIDE=y)\n");
#else
    printk("Command line: %s\n", saved_cmdline);
#endif

Other than that it is fine for me



> +}
> +
> +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 v2 2/2] xen/common: add keyhandler to show Xen command line

[dmukhin]

On Fri, Aug 07, 2026 at 05:44:41PM -0700, Stefano Stabellini wrote:
> On Mon, 3 Aug 2026, 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 (e.g. 'xl' is not available in dom0).
> > 
> > Add new keyhander 'X' to do command line printout.
> > 
> > Signed-off-by: Denis Mukhin <dmukhin@ford.com>
> > ---
> > v1: https://lore.kernel.org/xen-devel/20260730061459.2702672-2-dmukhin@ford.com/
> > 
> > Changes since v1:
> > - moved implementation into kernel.c
> > ---
> >  xen/common/kernel.c | 16 ++++++++++++++++
> >  1 file changed, 16 insertions(+)
> > 
> > diff --git a/xen/common/kernel.c b/xen/common/kernel.c
> > index d1bef9ac2b2b..9f334c92e3ab 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 +506,21 @@ 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);
> > +    printk("Command line: %s\n", saved_cmdline);
> 
> if CONFIG_CMDLINE_OVERRIDE is defined, saved_cmdline is empty. We could
> at least do this:
> 
> #ifdef CONFIG_CMDLINE_OVERRIDE
>     printk("Bootloader command line ignored (CONFIG_CMDLINE_OVERRIDE=y)\n");
> #else
>     printk("Command line: %s\n", saved_cmdline);
> #endif

Actually, CONFIG_CMDLINE can set the built-in command line which can be
non-empty.

Perhaps, something like this:

    printk("Command line (built-in): %s\n", opt_builtin_cmdline);
#ifndef CONFIG_CMDLINE_OVERRIDE
    printk("Command line: %s\n", saved_cmdline);
#endif

What do you think?

> 
> Other than that it is fine for me
> 
> 
> 
> > +}
> > +
> > +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 v2 2/2] xen/common: add keyhandler to show Xen command line

[Oleksii Kurochko]

On 8/8/26 4:38 AM, dmukhin@ford.com wrote:
> On Fri, Aug 07, 2026 at 05:44:41PM -0700, Stefano Stabellini wrote:
>> On Mon, 3 Aug 2026, 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 (e.g. 'xl' is not available in dom0).
>>>
>>> Add new keyhander 'X' to do command line printout.
>>>
>>> Signed-off-by: Denis Mukhin <dmukhin@ford.com>
>>> ---
>>> v1: https://lore.kernel.org/xen-devel/20260730061459.2702672-2-dmukhin@ford.com/
>>>
>>> Changes since v1:
>>> - moved implementation into kernel.c
>>> ---
>>>   xen/common/kernel.c | 16 ++++++++++++++++
>>>   1 file changed, 16 insertions(+)
>>>
>>> diff --git a/xen/common/kernel.c b/xen/common/kernel.c
>>> index d1bef9ac2b2b..9f334c92e3ab 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 +506,21 @@ 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);
>>> +    printk("Command line: %s\n", saved_cmdline);
>>
>> if CONFIG_CMDLINE_OVERRIDE is defined, saved_cmdline is empty. We could
>> at least do this:
>>
>> #ifdef CONFIG_CMDLINE_OVERRIDE
>>      printk("Bootloader command line ignored (CONFIG_CMDLINE_OVERRIDE=y)\n");
>> #else
>>      printk("Command line: %s\n", saved_cmdline);
>> #endif
> 
> Actually, CONFIG_CMDLINE can set the built-in command line which can be
> non-empty.
> 
> Perhaps, something like this:
> 
>      printk("Command line (built-in): %s\n", opt_builtin_cmdline);
> #ifndef CONFIG_CMDLINE_OVERRIDE
>      printk("Command line: %s\n", saved_cmdline);
> #endif
> 
> What do you think?

Considering that opt_builtin_cmdline is declared as __initconst won't it 
be an issue to print it in non-init function (show_hypervisor_info())? 
In other words, I expected that __init section should be freed at some 
point and I expect that it should be data abort here.

~ Oleksii

Re: [PATCH v2 2/2] xen/common: add keyhandler to show Xen command line

[Roger Pau Monné]

On Fri, Aug 07, 2026 at 07:38:28PM -0700, dmukhin@ford.com wrote:
> On Fri, Aug 07, 2026 at 05:44:41PM -0700, Stefano Stabellini wrote:
> > On Mon, 3 Aug 2026, 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 (e.g. 'xl' is not available in dom0).
> > > 
> > > Add new keyhander 'X' to do command line printout.
> > > 
> > > Signed-off-by: Denis Mukhin <dmukhin@ford.com>
> > > ---
> > > v1: https://lore.kernel.org/xen-devel/20260730061459.2702672-2-dmukhin@ford.com/
> > > 
> > > Changes since v1:
> > > - moved implementation into kernel.c
> > > ---
> > >  xen/common/kernel.c | 16 ++++++++++++++++
> > >  1 file changed, 16 insertions(+)
> > > 
> > > diff --git a/xen/common/kernel.c b/xen/common/kernel.c
> > > index d1bef9ac2b2b..9f334c92e3ab 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 +506,21 @@ 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);
> > > +    printk("Command line: %s\n", saved_cmdline);
> > 
> > if CONFIG_CMDLINE_OVERRIDE is defined, saved_cmdline is empty. We could
> > at least do this:
> > 
> > #ifdef CONFIG_CMDLINE_OVERRIDE
> >     printk("Bootloader command line ignored (CONFIG_CMDLINE_OVERRIDE=y)\n");
> > #else
> >     printk("Command line: %s\n", saved_cmdline);
> > #endif
> 
> Actually, CONFIG_CMDLINE can set the built-in command line which can be
> non-empty.
> 
> Perhaps, something like this:
> 
>     printk("Command line (built-in): %s\n", opt_builtin_cmdline);
> #ifndef CONFIG_CMDLINE_OVERRIDE
>     printk("Command line: %s\n", saved_cmdline);
> #endif

Aside from the remark from Oleksii, can you use IS_ENABLED() here
instead of preprocessor conditionals?

Thanks, Roger.