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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3b61ca4d48bsm14457627f8f.62.2025.07.22.14.53.43 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 22 Jul 2025 14:53:43 -0700 (PDT) Date: Tue, 22 Jul 2025 22:53:40 +0100 From: David Laight To: "H. Peter Anvin" Cc: Oleg Nesterov , Ingo Molnar , Peter Zijlstra , Thomas Gleixner , Borislav Petkov , Dave Hansen , "Li,Rongqing" , Steven Rostedt , linux-kernel@vger.kernel.org, x86@kernel.org Subject: Re: [PATCH] x86/math64: handle #DE in mul_u64_u64_div_u64() Message-ID: <20250722225340.1b0b29fe@pumpkin> In-Reply-To: References: <20250721130422.GA31640@redhat.com> <20250721192053.58843751@pumpkin> <20250722105034.GA2845@redhat.com> <20250722130947.0c97c96a@pumpkin> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Tue, 22 Jul 2025 09:53:21 -0700 "H. Peter Anvin" wrote: ... > > Note that -1 for division by zero (not necessarily for overflow) follows from > most natural division algorithms, and so architectures which don't trap > on division overflow tend to behave that way. Do you think that just returning ~0 is enough (maybe with a WARN_ONCE) ? Code can check for it and the number of false positives would be minimal. I have wondered about having muldiv(a, b, c, d, errval) that returns 'errval' if '(a * b + c)/ d' overflows or if 'd' is zero. Otherwise you are trying to return too many values from a function (esp. on 32bit). 'asm goto' or 'flags' will only work for x86_64, starts being a real mess for the generic code. (Time for a language feature that lets the caller see modifications to a parameter that is passed by value...) David