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

[Xiaoyao Li <xiaoyao.li@intel.com>]

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.