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Peter Anvin" CC: Usama Arif , david.laight.linux@gmail.com, 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: =?US-ASCII?Q?Re=3A_=5BPATCH_v2=5D_x86/lib=3A_Use_REP_MOVSB_for?= =?US-ASCII?Q?_large_memcpy=28=29_on_ERMS_CPUs_without_FSRM?= User-Agent: K-9 Mail for Android In-Reply-To: References: <20261104222806.1826508-1-usama.arif@linux.dev> <719584E3-A730-474D-9061-BFF35EA4B02B@zytor.com> Message-ID: <09B273E6-9146-4489-BC46-E7455CD8CA16@alien8.de> 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=utf-8 Content-Transfer-Encoding: quoted-printable On November 10, 2026 8:59:02 PM UTC, Linus Torvalds wrote: >On Tue, 10 Nov 2026 at 12:18, H=2E Peter Anvin wrote: >> >> On November 10, 2026 9:53:07 AM PST, Linus Torvalds wrote: >> >Let's not do that=2E >> > >> >Putting magic things like RET in alternatives is just a bad bad idea= =2E >> > >> > Linus >> >> Why is that, *given that it is in assembly*? > >It's assembly, but it's assembly that potentially gets rewritten=2E >objtool already knows about __x86_return_thunk being special (although >I think it's the _target_, not the call site, it cares about)=2E > >I would like to not have magic in stuff that is in the alternatives >section=2E We've had issues with relocations etc before=2E > >That said, grepping a bit for this, it turns out we already seem to be >doing that in memmove_64=2ES, although it was hidden from my first grep: > > #define CHECK_LEN cmp $0x20, %rdx; jb 1f > #define MEMMOVE_BYTES movq %rdx, %rcx; rep movsb; RET > =2ELmemmove_begin_forward: > ALTERNATIVE_2 __stringify(CHECK_LEN), \ > __stringify(CHECK_LEN; MEMMOVE_BYTES), X86_FEATURE_= ERMS, \ > __stringify(MEMMOVE_BYTES), X86_FEATURE_FSRM > >which is pretty disgusting=2E But I guess that train has passed=2E=2E > > Linus Yah, I never liked this piece of gunk=2E Lemme give it a fresh look when t= hings relax a bit=2E=2E=2E --=20 Small device=2E Typos and formatting crap