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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4bb55ac1159sm64869345e9.4.2026.11.13.08.03.01 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 13 Nov 2026 08:03:01 -0800 (PST) Date: Fri, 13 Nov 2026 16:03:00 +0000 From: David Laight To: Usama Arif Cc: Borislav Petkov , dave.hansen@linux.intel.com, dsterba@suse.com, hpa@zytor.com, linux-kernel@vger.kernel.org, mingo@redhat.com, terrelln@fb.com, tglx@kernel.org, x86@kernel.org, torvalds@linux-foundation.org, kas@kernel.org, hannes@cmpxchg.org, riel@surriel.com, shakeel.butt@linux.dev, kernel-team@meta.com Subject: Re: [PATCH v3 2/2] x86/lib: Use REP MOVSB for large memcpy() on ERMS CPUs without FSRM Message-ID: <20261113160300.3f0d0a37@pumpkin> In-Reply-To: <20261113141137.1113517-1-usama.arif@linux.dev> References: <20261111234608.GAavT-QFRo6EtvoL0y@fat_crate.local> <20261113141137.1113517-1-usama.arif@linux.dev> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Fri, 13 Nov 2026 06:11:36 -0800 Usama Arif wrote: > How about something like what I have added at the end, would that be more > acceptable in terms of maintainability? I was thinking more of something like: (written in the email so untested) static always_inline void *memcpy(void *dst, const void *src, size_t len) { void *orig_dst = dst; asm volatile( ".pushsection ".memcpy_calls\n" "\t.long . - 1f\n" ".popsection\n" "1:\trep movsb\n" "\tnop; nop" : "+c (len), "+D" (dst), "+S" (src) :: ASM_CALL_CONSTRAINT); return orig_dst; } An unused return value will be optimised away at all the call sites. That will work (maybe slowly) on all cpu. Then some code reads all the addresses from .memcpy_calls and, if necessary, patches the 5 bytes to a call to the appropriate function. It might be worth giving the code one or two scratch registers so that a short copy loop doesn't need to spill to stack (perhaps %rax and %rdx). It'll matter less for a longer copy loop. So you might have: _memcpy_erms: cmp $nnnn, %rcx jb _memcpy_words rep movsb RET _memcpy_words: cmpb $nnnn %rcx jb _memcpy_bytes mov %rax,%rcx or %rcx,%rdi or %rcx,%rsi test $7,%rcx jnz _memcpy_misaligned sub $8,%rax 1: mov %rcx,[%rsi] mov %rdx,8[%rsi] mov [%rdi],%rcx mov 8[%rdi],%rdx add $16,%rdi add $16,%rsi sub $16,%rax jnc 1b: jz 2f mov %rax,[%si] mov [%rdi]%rax] 2: RET That isn't the best 2-word loop by any means. And you may not care about misaligned pointers. etc David