From: David Laight <david.laight.linux@gmail.com>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
Usama Arif <usama.arif@linux.dev>,
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
Date: Tue, 10 Nov 2026 21:00:55 +0000 [thread overview]
Message-ID: <20261110210055.0ca9fc07@pumpkin> (raw)
In-Reply-To: <719584E3-A730-474D-9061-BFF35EA4B02B@zytor.com>
On Tue, 10 Nov 2026 12:17:51 -0800
"H. Peter Anvin" <hpa@zytor.com> wrote:
> On November 10, 2026 9:53:07 AM PST, Linus Torvalds <torvalds@linux-foundation.org> wrote:
> >On Wed, 4 Nov 2026 at 14:28, Usama Arif <usama.arif@linux.dev> 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
next prev parent reply other threads:[~2026-11-10 21:00 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-11-04 22:28 [PATCH v2] x86/lib: Use REP MOVSB for large memcpy() on ERMS CPUs without FSRM Usama Arif
2026-11-05 8:12 ` David Laight
2026-11-05 17:28 ` Usama Arif
2026-11-10 16:25 ` Kiryl Shutsemau
2026-11-10 17:06 ` David Laight
2026-11-11 13:02 ` Usama Arif
2026-11-11 12:58 ` Usama Arif
2026-11-10 17:53 ` Linus Torvalds
2026-11-10 19:33 ` David Laight
2026-11-10 20:17 ` H. Peter Anvin
2026-11-10 20:59 ` Linus Torvalds
2026-11-10 22:35 ` David Laight
2026-11-11 1:16 ` Borislav Petkov
2026-11-11 13:24 ` Usama Arif
2026-11-10 21:00 ` David Laight [this message]
2026-11-11 0:24 ` H. Peter Anvin
2026-11-11 13:04 ` Usama Arif
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