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X-CSE-ConnectionGUID: Upt8b2ggRUu1DNEC83O6vQ== X-CSE-MsgGUID: UMb9fyn8RqO0IgOxiEVtvg== X-IronPort-AV: E=McAfee;i="6800,10657,11664"; a="68180561" X-IronPort-AV: E=Sophos;i="6.21,208,1763452800"; d="scan'208";a="68180561" Received: from orviesa008.jf.intel.com ([10.64.159.148]) by fmvoesa113.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 07 Jan 2026 08:06:20 -0800 X-CSE-ConnectionGUID: jsxC5j/nSd2bDf+xbSz+3w== X-CSE-MsgGUID: jkLjd5tnTXGp12ZVi+Zprg== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.21,208,1763452800"; d="scan'208";a="203009398" Received: from aduenasd-mobl5.amr.corp.intel.com (HELO [10.125.109.145]) ([10.125.109.145]) by orviesa008-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 07 Jan 2026 08:06:20 -0800 Message-ID: Date: Wed, 7 Jan 2026 08:06:19 -0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] x86/split_lock: Handle unexpected split lock as fatal To: "Edgecombe, Rick P" , "Li, Xiaoyao" , "tglx@linutronix.de" , "mingo@redhat.com" , "x86@kernel.org" , "dave.hansen@linux.intel.com" , "bp@alien8.de" Cc: "hpa@zytor.com" , "Chatre, Reinette" , "kas@kernel.org" , "linux-kernel@vger.kernel.org" , "Qiang, Chenyi" , "Peng, Chao P" References: <20260107134955.3293885-1-xiaoyao.li@intel.com> <9fbbe3bc-0912-42af-b5c0-abda89e0b621@intel.com> <07cf71d75b25d8be00eac554244d2a2e15845fd5.camel@intel.com> From: Dave Hansen Content-Language: en-US Autocrypt: addr=dave.hansen@intel.com; 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charset=UTF-8 Content-Transfer-Encoding: 7bit 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. But I still want to hear more about why the enumeration is broken and can't be fixed.