linux-kernel - Re: [tip: x86/splitlock] Documentation/x86: Add buslock.rst

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Date:   Wed, 18 Aug 2021 09:59:49 +0800
From:   Xiaoyao Li <xiaoyao.li@...el.com>
To:     linux-kernel@...r.kernel.org, linux-tip-commits@...r.kernel.org
Cc:     Fenghua Yu <fenghua.yu@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tony Luck <tony.luck@...el.com>, x86@...nel.org
Subject: Re: [tip: x86/splitlock] Documentation/x86: Add buslock.rst

On 5/18/2021 10:44 PM, tip-bot2 for Fenghua Yu wrote:
...
> +
> +Software handling
> +=================
> +
> +The kernel #AC and #DB handlers handle bus lock based on the kernel
> +parameter "split_lock_detect". Here is a summary of different options:
> +
> ++------------------+----------------------------+-----------------------+
> +|split_lock_detect=|#AC for split lock		|#DB for bus lock	|
> ++------------------+----------------------------+-----------------------+
> +|off	  	   |Do nothing			|Do nothing		|
> ++------------------+----------------------------+-----------------------+
> +|warn		   |Kernel OOPs			|Warn once per task and |
> +|(default)	   |Warn once per task and	|and continues to run.  |
> +|		   |disable future checking	|			|
> +|		   |When both features are	|			|
> +|		   |supported, warn in #AC	|			|
> ++------------------+----------------------------+-----------------------+
> +|fatal		   |Kernel OOPs			|Send SIGBUS to user.	|
> +|		   |Send SIGBUS to user		|			|
> +|		   |When both features are	|			|
> +|		   |supported, fatal in #AC	|			|
> ++------------------+----------------------------+-----------------------+
> +

Hi all,

I'm wonder if using only one "split_lock_detect" parameter for those two 
features is good/correct.

In fact, split lock is just one type of bus lock. There are two types 
bus lock:
1) split lock, lock on WB memory across multiple cache lines;
2) lock on non-WB memory;

As current design, if both features are available, it only enables #AC 
for split lock either for "warn" or "fatal". Thus we cannot capture any 
bus lock due to 2) lock on non-WB memory.

Why not provide separate parameter for them? e.g., split_lock_detect and 
bus_lock_detect. Then they can be configured and enabled independently.