From: David Laight <david.laight.linux@gmail.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Kiryl Shutsemau <kas@kernel.org>,
Usama Arif <usama.arif@linux.dev>,
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
Date: Thu, 22 Oct 2026 09:49:58 +0100 [thread overview]
Message-ID: <20261022094958.13aeb5c0@pumpkin> (raw)
In-Reply-To: <CAHk-=wh+dyDPs2kp=KoUiCYdk2MBBqzTKYbrkAyOMV9Z1XNu0w@mail.gmail.com>
On Wed, 21 Oct 2026 08:18:25 -1000
Linus Torvalds <torvalds@linux-foundation.org> wrote:
> On Tue, 20 Oct 2026 at 23:55, Kiryl Shutsemau <kas@kernel.org> 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
>
next prev parent reply other threads:[~2026-10-22 8:50 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-10-13 10:04 Usama Arif
2026-10-15 10:39 ` Kiryl Shutsemau
2026-10-17 19:27 ` Usama Arif
2026-10-21 9:54 ` Kiryl Shutsemau
2026-10-21 18:18 ` Linus Torvalds
2026-10-22 8:49 ` David Laight [this message]
2026-10-17 21:42 ` David Laight
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