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Peter Anvin" , Dave Hansen CC: "Kirill A. Shutemov" , Andy Lutomirski , Thomas Gleixner , Ingo Molnar , Borislav Petkov , , "Peter Zijlstra" , Ard Biesheuvel , "Paul E. McKenney" , Josh Poimboeuf , Xiongwei Song , Xin Li , "Mike Rapoport (IBM)" , Brijesh Singh , Michael Roth , Tony Luck , Alexey Kardashevskiy , Alexander Shishkin , Jonathan Corbet , "Ingo Molnar" , Pawan Gupta , Daniel Sneddon , Kai Huang , Sandipan Das , Breno Leitao , Rick Edgecombe , "Alexei Starovoitov" , Hou Tao , Juergen Gross , Vegard Nossum , Kees Cook , Eric Biggers , Jason Gunthorpe , "Masami Hiramatsu (Google)" , "Andrew Morton" , Luis Chamberlain , Yuntao Wang , Rasmus Villemoes , Christophe Leroy , Tejun Heo , Changbin Du , Huang Shijie , Geert Uytterhoeven , Namhyung Kim , "Arnaldo Carvalho de Melo" , , , , References: <20250707080317.3791624-1-kirill.shutemov@linux.intel.com> <20250707080317.3791624-3-kirill.shutemov@linux.intel.com> <20250728201137.113d0f45@pumpkin> <98BE7799-DA6C-4267-9E03-E6F68263D250@zytor.com> <20250728203809.7bebf01c@pumpkin> From: Sohil Mehta In-Reply-To: <20250728203809.7bebf01c@pumpkin> Content-Type: text/plain; charset="UTF-8" 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U2FsdGVkX1+4D/EGP/XT/5BnQt8QR5bLJ1UCtg0FHvZNtxivI7Fdpc4UUk50pdgeLmEt8+CIp82ijx7rTKL5qLCMNtSjn0nPvOv0dOkwcAHnyCLRACROG1zyHEt2N2XOlZ0Imqk0Wd048rp986U+9pPVITyOoiOqK9ooOD3xUr6fM7rBvUHOMyYk4qNG0gxL0CECj+Twl0bpKcamPAJVsxHdOqgRaNbImd8wSR4hgw7e9Xj6t5ax7OGMgD27k1RPcsGOGGpBK0/JDKOh6lUci6fY3XR2SMBCDjjFt5icUSXjrswM4iLkNeq8w+lgjU0xWjQUpHkbnu/uoxXt3wu13Tw9eFn13XzMVZl1FlvdpC4kEGh6ZavtEr7CsFwbDV5nJOKGyqOtV75IsE1yfAWTk6Xj5Mk+nJSnkBoYtrN3cNe5zyEmb5OXIa5T87LuNlBACRUDEgpVUcOlAHWlYB3RABuy1ei2E5HZ/39bSu9sfMAQsbZiisSo6elPKUcR+KotSlAlYl4jSYjmHXzNF30g4UQxZwIxyWi9pllntrz9PZygbkNgjcWwX12v//pe8I0FdwSAqCvTu9fKzw+zgmlq9vQ2K984KZeaAwdLJxK8/KOWDww87zOe+w2X7NAJkGLhda0x68+GO1UUcCNVa6sCV8AnZ2FA627HWbFadVt5WPVi4eadLEhIfGvQWuBcBjcc/oRI7tVdlGoy2syglzoR/8+i0JVgeDsN3F9t+dJTNgOMcUZ9TcAH5N+frV6NC8KdV5H5qBG1BwkV3srVY9A0kkaEke/hsq1VH531O9r+K7aIY2/nRdPPZdPexFIm2PpBFJ4HY8sCQBfCZOz0fW3hGjys3VODhAjXlq1hLrtbH3EFKxunIqXcZfiClPcECJx3BUrQ/7X4fFQx37yEN9mQvltdC8QmSJPV1oTyhxmdqo+Hb0gdce1/RVJlM88bBUxPsM2E7Z9VMl9WhNn9ltT LALhqbTM 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>>> >>> Or just write a byte copy loop in C with (eg) barrier() inside it >>> to stop gcc converting it to memcpy(). >>> >>> David >> >> Great. It's rep movsb without any of the performance. > > And without the massive setup overhead that dominates short copies. > Given the rest of the code I'm sure a byte copy loop won't make > any difference to the overall performance. > Wouldn't it be better to introduce a generic mechanism than something customized for this scenario? PeterZ had suggested that inline memcpy could have more usages: https://lore.kernel.org/lkml/20241029113611.GS14555@noisy.programming.kicks-ass.net/ Is there a concern that the inline versions might get optimized into standard memcpy/memset calls by GCC? Wouldn't the volatile keyword prevent that? static __always_inline void *__inline_memcpy(void *to, const void *from, size_t len) { void *ret = to; asm volatile("rep movsb" : "+D" (to), "+S" (from), "+c" (len) : : "memory"); return ret; } static __always_inline void *__inline_memset(void *s, int v, size_t n) { void *ret = s; asm volatile("rep stosb" : "+D" (s), "+c" (n) : "a" ((uint8_t)v) : "memory"); return ret; }