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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-48e2584c3b0sm12297457f8f.57.2026.10.22.01.49.59 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 22 Oct 2026 01:50:00 -0700 (PDT) Date: Thu, 22 Oct 2026 09:49:58 +0100 From: David Laight To: Linus Torvalds Cc: Kiryl Shutsemau , Usama Arif , bp@alien8.de, 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, hannes@cmpxchg.org, riel@surriel.com, shakeel.butt@linux.dev, kernel-team@meta.com Subject: Re: [PATCH] x86/lib: Use REP MOVSB for large memcpy() on ERMS CPUs without FSRM Message-ID: <20261022094958.13aeb5c0@pumpkin> In-Reply-To: References: <20261017192727.2760919-1-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 Wed, 21 Oct 2026 08:18:25 -1000 Linus Torvalds wrote: > On Tue, 20 Oct 2026 at 23:55, Kiryl Shutsemau wrote: > > > > Moving MOVQ inside ALTERNATIVE_2 is ugly... > > Yeah, maybe it would make sense to make that function be > > movq %rdi, %rax > movq %rdx, %rcx > ALTERNATIVE "jmp memcpy_orig", "rep movsb", X86_FEATURE_FSRM > RET > > instead? No nop instructions at all, and both alternatives are two bytes. > > Then do any ERMS logic at the top of memcpy_orig, which now has the > registers ready for string instructions anyway. Is it possibly to just patch "rep movsb; RET" over whatever gets generated for memcpy_orig (after the two register moves) ? Then there are neither nops nor a jmp. David > > Hmm? > > Linus >