linux-kernel - Re: [PATCH v11 9/9] x86/cpu: Enable LASS by default during CPU initialization

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Message-ID: <f559e1fa-dfc6-435f-9337-30315891a394@intel.com>
Date: Fri, 31 Oct 2025 09:55:01 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: David Laight <david.laight.linux@...il.com>,
 Sohil Mehta <sohil.mehta@...el.com>
Cc: Andy Lutomirski <luto@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
 the arch/x86 maintainers <x86@...nel.org>,
 Dave Hansen <dave.hansen@...ux.intel.com>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Jonathan Corbet <corbet@....net>,
 Josh Poimboeuf <jpoimboe@...nel.org>,
 "Peter Zijlstra (Intel)" <peterz@...radead.org>,
 Ard Biesheuvel <ardb@...nel.org>, "Kirill A . Shutemov" <kas@...nel.org>,
 Xin Li <xin@...or.com>, David Woodhouse <dwmw@...zon.co.uk>,
 Sean Christopherson <seanjc@...gle.com>,
 Rick P Edgecombe <rick.p.edgecombe@...el.com>,
 Vegard Nossum <vegard.nossum@...cle.com>,
 Andrew Cooper <andrew.cooper3@...rix.com>,
 Randy Dunlap <rdunlap@...radead.org>,
 Geert Uytterhoeven <geert@...ux-m68k.org>, Kees Cook <kees@...nel.org>,
 Tony Luck <tony.luck@...el.com>,
 Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
 linux-doc@...r.kernel.org,
 Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
 linux-efi@...r.kernel.org
Subject: Re: [PATCH v11 9/9] x86/cpu: Enable LASS by default during CPU
 initialization

On 10/30/25 14:13, David Laight wrote:
>> Unfortunately, CONFIG_X86_VSYSCALL_EMULATION defaults to y. Also, the
>> default Vsyscall mode is XONLY. So even if vsyscalls are deprecated,
>> there is a non-zero possibility someone would complain about it.
> Presumably a command line parameter could be used to disable LASS
> in order to enable vsyscall emulation?
> 
> That might let LASS be enabled by default.

Sure... There are a million ways to skin this cat. That's the problem.

The compile switch is the smallest amount of code with the fewest
implications to ABI or documentation that we can muster. All I'm saying
is we should _start_ here, not _end_ here.

If anyone agrees with that approach, acks would be appreciated.