Introduce memcpy_nontemporal() for write-once copy sites that want a named non-temporal copy primitive. On x86_64, override the helper in arch/x86/include/asm/string_64.h using the usual self-macro pattern, next to the existing memcpy_flushcache() backend that memcpy_nontemporal() wraps. include/linux/string.h provides the generic memcpy_nontemporal() fallback as #define memcpy_nontemporal(dst, src, len) \ ((void)memcpy(dst, src, len)) instead of an inline wrapper, so architectures without a specialized backend keep the usual memcpy() FORTIFY coverage when the compiler can still see object sizes at the original call site. It also makes the memcpy_nontemporal() API uniformly void, matching memcpy_flushcache() and the x86 backend, so callers cannot accidentally depend on a return value on fallback architectures. memcpy_nontemporal() is only a copy primitive. It does not imply a drain or a publication barrier. Callers that use it before a producer-consumer or device-visible handoff must provide the required ordering at that handoff point. The immediate user is the ZONE_DEVICE template-copy path. It populates struct page descriptors in a write-once pattern, so a regular cached memcpy() can incur avoidable write-allocate traffic and cache pollution for data with little near-term reuse. Link: https://lore.kernel.org/20260831111638.76012-6-lizhe.67@bytedance.com Signed-off-by: Li Zhe <lizhe.67@bytedance.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Cc: Alistair Popple <apopple@nvidia.com> Cc: Arnd Bergmann <arnd@arndb.de> Cc: Balbir Singh <balbirs@nvidia.com> Cc: "Borislav Petkov (AMD)" <bp@alien8.de> Cc: Dave Hansen <dave.hansen@linux.intel.com> Cc: David Hildenbrand (Arm) <david@kernel.org> Cc: Ingo Molnar <mingo@redhat.com> Cc: Kees Cook <kees@kernel.org> Cc: Mike Rapoport (Microsoft) <rppt@kernel.org> Cc: Muchun Song <muchun.song@linux.dev>