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

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Message-ID: <20251030211318.74d90c3f@pumpkin>
Date: Thu, 30 Oct 2025 21:13:18 +0000
From: David Laight <david.laight.linux@...il.com>
To: 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 Thu, 30 Oct 2025 09:44:02 -0700
Sohil Mehta <sohil.mehta@...el.com> wrote:

> On 10/30/2025 8:45 AM, Andy Lutomirski wrote:
> > On Thu, Oct 30, 2025, at 1:40 AM, H. Peter Anvin wrote:  
> >> Legacy vsyscalls have been obsolete for how long now?  
> > 
> > A looooong time.
> > 
> > I would suggest defaulting LASS to on and *maybe* decoding just enough to log, once per boot, that a legacy vsyscall may have been attempted. It’s too bad that #GP doesn’t report the faulting address.
> >   
> 
> 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.

	David