[PATCH] documentation: Add nodebugmon for arm64 platforms

[PATCH] documentation: Add nodebugmon for arm64 platforms

[Anshuman Khandual]

Add an entry for nodebugmon which has been used on arm64 platforms.

Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: linux-doc@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
---
This patch applies on v6.15-rc2

 Documentation/admin-guide/kernel-parameters.txt | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index d9fd26b95b34..f4a313d6c0ab 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4085,6 +4085,13 @@
 			/sys/module/printk/parameters/console_suspend) to
 			turn on/off it dynamically.
 
+	nodebugmon
+			[HW,ARM64] Disable debug monitor
+			Disables the debug monitor exception handling on the platform
+			regardless whether breakpoints and watchpoints are programmed
+			or not. This prevents debug exceptions from being enabled via
+			MDSCR_EL1 register.
+
 	no_debug_objects
 			[KNL,EARLY] Disable object debugging
 
-- 
2.25.1

Re: [PATCH] documentation: Add nodebugmon for arm64 platforms

[Bagas Sanjaya]

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On Wed, Apr 16, 2025 at 11:57:06AM +0530, Anshuman Khandual wrote:
> +	nodebugmon
> +			[HW,ARM64] Disable debug monitor
> +			Disables the debug monitor exception handling on the platform
> +			regardless whether breakpoints and watchpoints are programmed
> +			or not. This prevents debug exceptions from being enabled via
> +			MDSCR_EL1 register.

The wording looks good, thanks!

Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>

-- 
An old man doll... just what I always wanted! - Clara

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Re: [PATCH] documentation: Add nodebugmon for arm64 platforms

[Anshuman Khandual]

On 4/16/25 11:57, Anshuman Khandual wrote:
> Add an entry for nodebugmon which has been used on arm64 platforms.
> 
> Cc: Jonathan Corbet <corbet@lwn.net>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: linux-doc@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Cc: linux-arm-kernel@lists.infradead.org
> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> ---
> This patch applies on v6.15-rc2
> 
>  Documentation/admin-guide/kernel-parameters.txt | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index d9fd26b95b34..f4a313d6c0ab 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -4085,6 +4085,13 @@
>  			/sys/module/printk/parameters/console_suspend) to
>  			turn on/off it dynamically.
>  
> +	nodebugmon
> +			[HW,ARM64] Disable debug monitor
> +			Disables the debug monitor exception handling on the platform
> +			regardless whether breakpoints and watchpoints are programmed
> +			or not. This prevents debug exceptions from being enabled via
> +			MDSCR_EL1 register.
> +
>  	no_debug_objects
>  			[KNL,EARLY] Disable object debugging
>  

Any updates on this ?

Re: [PATCH] documentation: Add nodebugmon for arm64 platforms

[Will Deacon]

On Wed, Apr 16, 2025 at 11:57:06AM +0530, Anshuman Khandual wrote:
> Add an entry for nodebugmon which has been used on arm64 platforms.
> 
> Cc: Jonathan Corbet <corbet@lwn.net>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: linux-doc@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Cc: linux-arm-kernel@lists.infradead.org
> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> ---
> This patch applies on v6.15-rc2
> 
>  Documentation/admin-guide/kernel-parameters.txt | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index d9fd26b95b34..f4a313d6c0ab 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -4085,6 +4085,13 @@
>  			/sys/module/printk/parameters/console_suspend) to
>  			turn on/off it dynamically.
>  
> +	nodebugmon
> +			[HW,ARM64] Disable debug monitor
> +			Disables the debug monitor exception handling on the platform
> +			regardless whether breakpoints and watchpoints are programmed
> +			or not. This prevents debug exceptions from being enabled via
> +			MDSCR_EL1 register.

Hmm. I appreciate the effort to document this, but I'm not sure that the
text above is really going to help anybody.

Firstly, this option goes hand-in-hand with a debugfs control
("debug_enabled") and so the two would ideally be documented together.

Secondly, I think the documentation should talk about the user-visible
effects that the control has... and that's where I get stuck! This has
been there since day 1 but I'm not really sure what it's useful for. I
_do_ remember needing it to use the DS-5 debugger back in the day, but
looking at what the option does (it mostly prevents the kernel from
touching MDSCR_EL1.{MDE,KDE} which effectively disables hardware
breakpoints and watchpoints for kernel/user and single-step for kernel)
I don't see why that's relevant for halting debug. The architecture says
that EDSCR.HDE can be used by the external debugger to enter debug state
and I don't see anything to suggest that MDSCR_EL1 settings interfere
with this.

Maybe the problem was that the external debugger left a load of live
breakpoint registers around after detaching, so this was a way to
suppress those? I don't remember :/

I wonder if we can remove this?

Will

Re: [PATCH] documentation: Add nodebugmon for arm64 platforms

[Anshuman Khandual]

On 4/29/25 21:57, Will Deacon wrote:
> On Wed, Apr 16, 2025 at 11:57:06AM +0530, Anshuman Khandual wrote:
>> Add an entry for nodebugmon which has been used on arm64 platforms.
>>
>> Cc: Jonathan Corbet <corbet@lwn.net>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: Catalin Marinas <catalin.marinas@arm.com>
>> Cc: Will Deacon <will@kernel.org>
>> Cc: Mark Rutland <mark.rutland@arm.com>
>> Cc: linux-doc@vger.kernel.org
>> Cc: linux-kernel@vger.kernel.org
>> Cc: linux-arm-kernel@lists.infradead.org
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
>> ---
>> This patch applies on v6.15-rc2
>>
>>  Documentation/admin-guide/kernel-parameters.txt | 7 +++++++
>>  1 file changed, 7 insertions(+)
>>
>> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
>> index d9fd26b95b34..f4a313d6c0ab 100644
>> --- a/Documentation/admin-guide/kernel-parameters.txt
>> +++ b/Documentation/admin-guide/kernel-parameters.txt
>> @@ -4085,6 +4085,13 @@
>>  			/sys/module/printk/parameters/console_suspend) to
>>  			turn on/off it dynamically.
>>  
>> +	nodebugmon
>> +			[HW,ARM64] Disable debug monitor
>> +			Disables the debug monitor exception handling on the platform
>> +			regardless whether breakpoints and watchpoints are programmed
>> +			or not. This prevents debug exceptions from being enabled via
>> +			MDSCR_EL1 register.
> 
> Hmm. I appreciate the effort to document this, but I'm not sure that the
> text above is really going to help anybody.
> 
> Firstly, this option goes hand-in-hand with a debugfs control
> ("debug_enabled") and so the two would ideally be documented together.

Completely agree on this, but could find an appropriate Documentation/
file listing arm64 platform relate debugfs interfaces. Just wondering
if we need to have such a file created in Documentation/arch/arm64 ?

> 
> Secondly, I think the documentation should talk about the user-visible
> effects that the control has... and that's where I get stuck! This has
> been there since day 1 but I'm not really sure what it's useful for. I

Hmm, alright. I tried to reason about what is being done in the kernel
but as you mentioned above, user visible effects were not very clearly
apparent. But having a kernel command line option without documenting
on what it really does seemed problematic as well.

> _do_ remember needing it to use the DS-5 debugger back in the day, but
> looking at what the option does (it mostly prevents the kernel from
> touching MDSCR_EL1.{MDE,KDE} which effectively disables hardware
> breakpoints and watchpoints for kernel/user and single-step for kernel)

Right, it is an system wide override regardless whether there are any
current users for these break/watch points.

> I don't see why that's relevant for halting debug. The architecture says
> that EDSCR.HDE can be used by the external debugger to enter debug state
> and I don't see anything to suggest that MDSCR_EL1 settings interfere
> with this.

ARM ARM DDI 0487 L.A states that EDSCR.HDE can be used to enable/disable
debug halting with OSLSR_EL1.OSLK = 1. Hence MDSCR_EL1.{MDE,KDE} controls
are not required to disable debug state in kernel.

> 
> Maybe the problem was that the external debugger left a load of live
> breakpoint registers around after detaching, so this was a way to
> suppress those? I don't remember :/

But that would be a bug in the HW, not an expected behaviour - is not it ?

> 
> I wonder if we can remove this?

Yes, we could probably remove both 'nodebugmon' kernel command line option
and 'debug_enabled' debug fs control if EDSCR.HDE is the controller of the
debug state for external debuggers.