linux-kernel - Re: [RFC PATCH 25/28] x86: Use PIE codegen for the core kernel

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Message-ID: <CAFULd4ZZxrJvJ9gF5tC-m-tmcDMvVM3te4xc7vnbF_OFU0D2=A@mail.gmail.com>
Date: Sun, 6 Oct 2024 10:06:33 +0200
From: Uros Bizjak <ubizjak@...il.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: "H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>, Ard Biesheuvel <ardb+git@...gle.com>, 
	linux-kernel@...r.kernel.org, x86@...nel.org, 
	Andy Lutomirski <luto@...nel.org>, Peter Zijlstra <peterz@...radead.org>, Dennis Zhou <dennis@...nel.org>, 
	Tejun Heo <tj@...nel.org>, Christoph Lameter <cl@...ux.com>, 
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Paolo Bonzini <pbonzini@...hat.com>, 
	Vitaly Kuznetsov <vkuznets@...hat.com>, Juergen Gross <jgross@...e.com>, 
	Boris Ostrovsky <boris.ostrovsky@...cle.com>, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Arnd Bergmann <arnd@...db.de>, 
	Masahiro Yamada <masahiroy@...nel.org>, Kees Cook <kees@...nel.org>, 
	Nathan Chancellor <nathan@...nel.org>, Keith Packard <keithp@...thp.com>, 
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	Ian Rogers <irogers@...gle.com>, Adrian Hunter <adrian.hunter@...el.com>, 
	Kan Liang <kan.liang@...ux.intel.com>, linux-doc@...r.kernel.org, 
	linux-pm@...r.kernel.org, kvm@...r.kernel.org, xen-devel@...ts.xenproject.org, 
	linux-efi@...r.kernel.org, linux-arch@...r.kernel.org, 
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	llvm@...ts.linux.dev
Subject: Re: [RFC PATCH 25/28] x86: Use PIE codegen for the core kernel

On Sun, Oct 6, 2024 at 2:00 AM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Sat, 5 Oct 2024 at 16:37, H. Peter Anvin <hpa@...or.com> wrote:
> >
> > Sadly, that is not correct; neither gcc nor clang uses lea:
>
> Looking around, this may be intentional. At least according to Agner,
> several cores do better at "mov immediate" compared to "lea".
>
> Eg a RIP-relative LEA on Zen 2 gets a throughput of two per cycle, but
> a "MOV r,i" gets four. That got fixed in Zen 3 and later, but
> apparently Intel had similar issues (Ivy Bridge: 1 LEA per cycle, vs 3
> "mov i,r". Haswell is 1:4).

Yes, this is the case. I just missed your reply when replying to
Peter's mail with a not so precise answer.

Uros.