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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-4983284c9b1sm6460691f8f.6.2026.11.10.13.00.56 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 10 Nov 2026 13:00:57 -0800 (PST) Date: Tue, 10 Nov 2026 21:00:55 +0000 From: David Laight To: "H. Peter Anvin" Cc: Linus Torvalds , Usama Arif , bp@alien8.de, dave.hansen@linux.intel.com, dsterba@suse.com, linux-kernel@vger.kernel.org, mingo@redhat.com, terrelln@fb.com, tglx@kernel.org, x86@kernel.org, kas@kernel.org, hannes@cmpxchg.org, riel@surriel.com, shakeel.butt@linux.dev, kernel-team@meta.com Subject: Re: [PATCH v2] x86/lib: Use REP MOVSB for large memcpy() on ERMS CPUs without FSRM Message-ID: <20261110210055.0ca9fc07@pumpkin> In-Reply-To: <719584E3-A730-474D-9061-BFF35EA4B02B@zytor.com> References: <20261104222806.1826508-1-usama.arif@linux.dev> <719584E3-A730-474D-9061-BFF35EA4B02B@zytor.com> 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 Tue, 10 Nov 2026 12:17:51 -0800 "H. Peter Anvin" wrote: > On November 10, 2026 9:53:07 AM PST, Linus Torvalds wrote: > >On Wed, 4 Nov 2026 at 14:28, Usama Arif wrote: > >> > >> + ALTERNATIVE __stringify(cmpq $0x20, %rcx; jb .Lhandle_tail), \ > >> + __stringify(rep movsb; RET), X86_FEATURE_FSRM > > > >Let's not do that. > > > >Putting magic things like RET in alternatives is just a bad bad idea. > > > > Linus > > Why is that, *given that it is in assembly*? > > For C (or Rust, or C++, ...) it is obviously bad chicken. Because RET is a horrid sequence subject to a lot of patching to avoid multiple speculative execution issues. If is were just the 0xc3 opcode byte it wouldn't be an issue. David