[v4] xen/common: add keyhandler to show Xen command line

[PATCH v4] xen/common: add keyhandler to show Xen command line

Posted by dmukhin@ford.com 1 day, 11 hours ago

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 v3:
- print built-in command line too
- change handler to print command line only

Changes since v2:
- account for CONFIG_CMDLINE_OVERRIDE case

v3: https://lore.kernel.org/xen-devel/20260810230359.671587-3-dmukhin@ford.com/
CI: https://gitlab.com/xen-project/people/dmukhin/xen/-/pipelines/2751523722
---
 xen/common/kernel.c | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/xen/common/kernel.c b/xen/common/kernel.c
index d1bef9ac2b2b..8a43e4421002 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 +506,37 @@ static int __init cf_check param_init(void)
 __initcall(param_init);
 #endif
 
+static void cf_check show_cmdline(unsigned char key)
+{
+    const char *builtin_cmdline = CONFIG_CMDLINE;
+    const char *cmdline;
+
+    printk("'%c' pressed -> showing hypervisor command line\n", key);
+
+    if ( builtin_cmdline[0] )
+        cmdline = builtin_cmdline;
+    else
+        cmdline = "<NULL>";
+
+    printk("Built-in command line: %s\n", cmdline);
+
+    if ( IS_ENABLED(CONFIG_CMDLINE_OVERRIDE) )
+        cmdline = "<NULL> (CONFIG_CMDLINE_OVERRIDE=y)";
+    else
+        cmdline = saved_cmdline;
+
+    printk("Command line: %s\n", cmdline);
+}
+
+static int __init cf_check misc_init(void)
+{
+    register_keyhandler('X', show_cmdline,
+                        "show hypervisor command line", 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 v4] xen/common: add keyhandler to show Xen command line

Posted by Oleksii Kurochko 1 day ago


On 8/11/26 11:09 PM, dmukhin@ford.com wrote:
> Currently there's no way to print Xen command line on the emergency
> console for debugging purposes when 'xl' is not unavailable.

It looks like 'not' should be dropped. 'unavailable' covers 'not' itself.

> 
> Add new keyhander 'X' to do command line printout.
> 
> Signed-off-by: Denis Mukhin <dmukhin@ford.com>
> ---
> Changes since v3:
> - print built-in command line too
> - change handler to print command line only
> 
> Changes since v2:
> - account for CONFIG_CMDLINE_OVERRIDE case
> 
> v3: https://lore.kernel.org/xen-devel/20260810230359.671587-3-dmukhin@ford.com/
> CI: https://gitlab.com/xen-project/people/dmukhin/xen/-/pipelines/2751523722
> ---
>   xen/common/kernel.c | 32 ++++++++++++++++++++++++++++++++
>   1 file changed, 32 insertions(+)
> 
> diff --git a/xen/common/kernel.c b/xen/common/kernel.c
> index d1bef9ac2b2b..8a43e4421002 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 +506,37 @@ static int __init cf_check param_init(void)
>   __initcall(param_init);
>   #endif
>   
> +static void cf_check show_cmdline(unsigned char key)
> +{
> +    const char *builtin_cmdline = CONFIG_CMDLINE;
> +    const char *cmdline;
> +
> +    printk("'%c' pressed -> showing hypervisor command line\n", key);
> +
> +    if ( builtin_cmdline[0] )
> +        cmdline = builtin_cmdline;
> +    else
> +        cmdline = "<NULL>";
> +
> +    printk("Built-in command line: %s\n", cmdline);
> +
> +    if ( IS_ENABLED(CONFIG_CMDLINE_OVERRIDE) )
> +        cmdline = "<NULL> (CONFIG_CMDLINE_OVERRIDE=y)";

Won't be better to use <ignored> instead of <NULL>? Also it reflects the 
comment (But if CONFIG_CMDLINE_OVERRIDE is set to y, @cmdline will be 
ignored.) above cmdline_parse() function better.

> +    else
> +        cmdline = saved_cmdline;

IIUC, saved_cmdline could be empty and I think we should take that into 
account too:

     if ( IS_ENABLED(CONFIG_CMDLINE_OVERRIDE) )
         cmdline = "<ignored> (CONFIG_CMDLINE_OVERRIDE=y)";
     else if ( saved_cmdline[0] )
         cmdline = saved_cmdline;
     else
         cmdline = "<NULL>";

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

Last argument is declared as bool so I think it will be better to use 
false here.

~ Oleksii

Re: [PATCH v4] xen/common: add keyhandler to show Xen command line

Posted by Jan Beulich 1 day ago

On 11.08.2026 23:09, dmukhin@ford.com wrote:
> @@ -505,6 +506,37 @@ static int __init cf_check param_init(void)
>  __initcall(param_init);
>  #endif
>  
> +static void cf_check show_cmdline(unsigned char key)
> +{
> +    const char *builtin_cmdline = CONFIG_CMDLINE;

Maybe compilers manage to optimize this to the equivalent of

    static const char builtin_cmdline[] = CONFIG_CMDLINE;

yet even then I see no reason why it wouldn't want spelling that way anyway.

Plus this raises the question: Why would we need two instances of the string
in the Xen image? If you need the literal at runtime, re-use
opt_builtin_cmdline[] by dropping __initconst from it.

> +    const char *cmdline;
> +
> +    printk("'%c' pressed -> showing hypervisor command line\n", key);
> +
> +    if ( builtin_cmdline[0] )
> +        cmdline = builtin_cmdline;
> +    else
> +        cmdline = "<NULL>";
> +
> +    printk("Built-in command line: %s\n", cmdline);

What use is this line when <NULL> is shown?

Jan