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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4b90f11172esm85799055e9.12.2026.11.08.12.30.43 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 08 Nov 2026 12:30:44 -0800 (PST) Date: Sun, 8 Nov 2026 20:30:42 +0000 From: David Laight To: Arnd Bergmann Cc: Ingo Molnar , Arnd Bergmann , Richard Weinberger , Anton Ivanov , Johannes Berg , Thomas Gleixner , Borislav Petkov , Dave Hansen , "H. Peter Anvin" , Peter Zijlstra , Will Deacon , Boqun Feng , Gary Guo , Yury Norov , Rasmus Villemoes , Boris Ostrovsky , Alexander Usyskin , Tony Nguyen , Przemek Kitszel , x86@kernel.org, linux-kernel@vger.kernel.org, linux-um@lists.infradead.org Subject: Re: [PATCH 3/7] x86: make CX8 usage unconditional Message-ID: <20261108203042.7e36f15b@pumpkin> In-Reply-To: <20261105163440.2381661-4-arnd@kernel.org> References: <20261105163440.2381661-1-arnd@kernel.org> <20261105163440.2381661-4-arnd@kernel.org> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) X-Archived-By: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.9.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20261108_123046_477243_99802F07 X-CRM114-Status: GOOD ( 12.28 ) X-Spam-Score: -2.1 (--) X-BeenThere: linux-um@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-um" X-Spam-Status: No, score=-4.3 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,FREEMAIL_FORGED_FROMDOMAIN, FREEMAIL_FROM,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_2 autolearn=no autolearn_force=no version=4.0.1 X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On Thu, 5 Nov 2026 17:34:34 +0100 Arnd Bergmann wrote: > From: Arnd Bergmann > > All supported CPUs now provide the cmpxchg8b instruction, so remove > all compile-time and runtime checks for its presence and the fallback > implementation, to assume it just works. Is it worth doing a run-time check early on any (trying) to output a message and fail to boot? There are probably a few other things the kernel could rely on, but probably ought to do a check 'just in case'. One that springs to mind is whether 'find first set' leaves the dest unchanged for zero. There is a comment that 'at least some 486 didn't' but assuming it can make code smaller. (Basically you can do a double-width operation by putting the result from the low word in the destination before doing the high word.) But it would be nice to have one test case during boot. David