linux-kernel - Re: [PATCH 2/2] x86/microcode: Add microcode loader debugging functionality

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Message-ID: <82cdc358-3551-4c8b-a387-0e9da4668b44@intel.com>
Date: Mon, 11 Aug 2025 20:04:58 -0700
From: Sohil Mehta <sohil.mehta@...el.com>
To: Borislav Petkov <bp@...nel.org>, X86 ML <x86@...nel.org>
CC: LKML <linux-kernel@...r.kernel.org>, "Borislav Petkov (AMD)"
	<bp@...en8.de>
Subject: Re: [PATCH 2/2] x86/microcode: Add microcode loader debugging
 functionality

On 8/9/2025 2:42 AM, Borislav Petkov wrote:

> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 7c095177da85..fc002b1a9f57 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -3770,6 +3770,13 @@
>  	microcode=      [X86] Control the behavior of the microcode loader.
>  	                Available options, comma separated:
>  
> +			dbg - Format: <bool>

Since this is all new code, would it be better to use the expanded form
of dbg everywhere?

s/dbg/debug/
s/MICROCODE_DBG/MICROCODE_DEBUG
s/ucode_dbg/ucode_debug

Also, I didn't understand the "Format: <bool>".

Isn't the usage similar to force_minrev? So microcode=debug should be
enough, right?

> +			enable debugging mode when run in a guest
> +
> +			base_rev=X - with <X> with format: <u32>
> +			Set the base microcode revision of each thread when in
> +			debug mode.
> +
>  			force_minrev
>  			Enable or disable the microcode minimal revision
>   			enforcement for the runtime microcode loader.


Slight preference for a tabbed description:

	debug
		enable debugging mode when run in a guest

	base_rev=X - with <X> with format: <u32>
		Set the base microcode revision of each thread when in
		debug mode.

	force_minrev
		Enable or disable the microcode minimal revision
		enforcement for the runtime microcode loader.


> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 58d890fe2100..462bf03aeda5 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -1360,6 +1360,18 @@ config MICROCODE_LATE_FORCE_MINREV
>  
>  	  If unsure say Y.
>  
> +config MICROCODE_DBG
> +	bool "Enable microcode loader debugging"
> +	default n

Isn't default n redundant? I am fine with keeping it to make it obvious.

> +	depends on MICROCODE

Should we also include a dependency on DEBUG_KERNEL?

> +	help
> +	  Enable code which allows for debugging the microcode loader in
> +	  a guest. Meaning the patch loading is simulated but everything else
> +	  related to patch parsing and handling is done as on baremetal with
> +	  the purpose of debugging solely the software side of things.
> +
> +	  You almost certainly want to say n here.
> +