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

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Date:   Wed, 18 Aug 2021 15:36:06 +0000
From:   Fenghua Yu <fenghua.yu@...el.com>
To:     Xiaoyao Li <xiaoyao.li@...el.com>
Cc:     linux-kernel@...r.kernel.org, linux-tip-commits@...r.kernel.org,
        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 Wed, Aug 18, 2021 at 09:59:49AM +0800, Xiaoyao Li wrote:
> On 5/18/2021 10:44 PM, tip-bot2 for Fenghua Yu wrote:
> 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.

#AC for split lock is a model specific feature and only available on limited
(and legacy) platforms. #DB for bus lock is an architectural feature and will
replace #AC for split lock in future platforms. The platforms that support
both of them are very rare (maybe only one AFAIK). Adding two parameters makes
code and usage complex while only one platform may get benefit in reality.

Thanks.

-Fenghua