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

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Message-ID: <bf7c04fc-73dc-4abf-a5be-19ad4d792c09@intel.com>
Date: Thu, 8 Jan 2026 10:19:57 +0800
From: Xiaoyao Li <xiaoyao.li@...el.com>
To: Dave Hansen <dave.hansen@...el.com>,
 "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
 "tglx@...utronix.de" <tglx@...utronix.de>,
 "mingo@...hat.com" <mingo@...hat.com>, "x86@...nel.org" <x86@...nel.org>,
 "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
 "bp@...en8.de" <bp@...en8.de>
Cc: "hpa@...or.com" <hpa@...or.com>,
 "Chatre, Reinette" <reinette.chatre@...el.com>,
 "kas@...nel.org" <kas@...nel.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "Qiang, Chenyi" <chenyi.qiang@...el.com>,
 "Peng, Chao P" <chao.p.peng@...el.com>
Subject: Re: [PATCH v2] x86/split_lock: Handle unexpected split lock as fatal

On 1/8/2026 12:06 AM, Dave Hansen wrote:
> On 1/7/26 07:24, Edgecombe, Rick P wrote:
>>> 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.
>> Can you clarify your feelings on BUG_ON()'s? I was under the impression
>> that new ones were basically banned, and we should WARN() here to try
>> to keep running.
>>
>> Unless we could claim that continuing would destroy something or other
>> situation a user would never want.
> 
> I'm conflicted about BUG_ON() here. It's a pretty nasty thing to be
> sending exceptions that the kernel doesn't expect. x86 exception
> handling is "fun" and has lots of sharp edges. There are absolutely
> windows where the kernel can not recover from exceptions if they happen
> in there. The real questions is why the kernel should even try to
> recover if it's faced with a borderline malicious hypervisor or CPU so
> buggy it's throwing unexpected exceptions.
> 
> On the other hand, in practice, this particular code path is from
> userspace and a BUG_ON() is an instant DoS.
> 
> Balancing all that, a WARN_ON_ONCE() with panic_on_warn=1 is probably
> the best course of action here.

Given that WARN_ON_ONCE() is 100% triggerable in TDX guest with a 
default host (CONFIG_X86_BUS_LOCK_DETECT=y && sld_state != sld_off) , is 
it OK to add it?

> But I still want to hear more about why the enumeration is broken and
> can't be fixed.

please see my reply to your original ask.