linux-kernel - Re: [PATCH v2] x86/split_lock: Handle unexpected split lock as fatal

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Message-ID: <9fbbe3bc-0912-42af-b5c0-abda89e0b621@intel.com>
Date: Wed, 7 Jan 2026 07:19:19 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: Xiaoyao Li <xiaoyao.li@...el.com>,
 Dave Hansen <dave.hansen@...ux.intel.com>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, x86@...nel.org
Cc: "H. Peter Anvin" <hpa@...or.com>, reinette.chatre@...el.com,
 Kiryl Shutsemau <kas@...nel.org>, rick.p.edgecombe@...el.com,
 linux-kernel@...r.kernel.org, chenyi.qiang@...el.com, chao.p.peng@...el.com
Subject: Re: [PATCH v2] x86/split_lock: Handle unexpected split lock as fatal

On 1/7/26 05:49, Xiaoyao Li wrote:
> +	/*
> +	 * If #AC occurs on split lock without X86_FEATURE_SPLIT_LOCK_DETECT
> +	 * the kernel cannot handle it by disabling the detection. Treat it as
> +	 * fatal regardless of the sld_state.
> +	 */
> +	if (!cpu_feature_enabled(X86_FEATURE_SPLIT_LOCK_DETECT))
> +		return true;

If #AC occurs on split lock without X86_FEATURE_SPLIT_LOCK_DETECT, that
sounds more like a naughty hypervisor or buggy CPU that deserves a
BUG_ON() rather than a situation where the kernel wants to move merrily
along.

This also needs an explanation in the changelog about _why_
X86_FEATURE_SPLIT_LOCK_DETECT isn't set and can't be set. It needs to
explain why enumeration is not present *AND* is impossible to add.