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Tue, 3 Jun 2025 18:32:57 +0000 Message-ID: <0d9d3317-ad0f-4080-9b24-230b76006d91@intel.com> Date: Tue, 3 Jun 2025 11:32:52 -0700 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v8 0/6] Introduce CET supervisor state support To: Chao Gao CC: Dave Hansen , Sean Christopherson , , , , , , , , , , , , Dave Hansen , Eric Biggers , "H. Peter Anvin" , Ingo Molnar , Kees Cook , Maxim Levitsky , Mitchell Levy , "Nikolay Borisov" , Oleg Nesterov , Sohil Mehta , Stanislav Spassov , "Vignesh Balasubramanian" References: <20250522151031.426788-1-chao.gao@intel.com> <434810d9-1b36-496d-a10d-41c6c068375e@intel.com> Content-Language: en-US From: "Chang S. 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And, in the Right, AMX instructions can be executed when XFD is disarmed. But in this case, it's inside a signal handler. On sigreturn, XTILE_DATA will be reloaded with the init state, since fpstate::user_xfeatures[18] is zero. > "flipped" scenario you mentioned, task #2 might receive an extra #NM, after > which its fpstate would be re-allocated (although the size won't increase > further). Yes. > So, for well-behaved tasks that never use AMX, there is no impact; tasks > that use AMX may receive extra #NM. There won't be any unexpected #PF, > memory corruption, or kernel panic. A signal handler is expected to stay within the bounds of async-signal-safe functions, so using AMX in that context is highly unlikely in practice. While the issue has existed, its real-world impact appears quite minimal in my view. Thanks, Chang