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

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Message-ID: <e3215a62-335f-463b-98d1-de95e5859b9d@intel.com>
Date: Thu, 30 Oct 2025 10:24:24 -0700
From: Sohil Mehta <sohil.mehta@...el.com>
To: 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>
CC: 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 10/30/2025 9:53 AM, Andy Lutomirski wrote:

>> On the other hand, supporting EMULATE mode during a #GP is a bit tricky,
>> which isn't worth the effort.
> 
> I would say it's definitely worth the effort, but it probably does make sense to get the rest of the series in a mergeable condition such that it only works with vsyscall=none.

I meant the full emulation mode where the Vsyscall page is readable. It
is only available via vsyscall=emulate. No one should be using that one,
right?

I thought you and Linus agreed on removing EMULATE mode completely:
https://lore.kernel.org/all/CALCETrXHJ7837+cmahg-wjR3iRHbDJ6JtVGaoDFC4dx-L8r8OA@mail.gmail.com/

I agree that it would be worthwhile (and relatively easy) to support the
execute (XONLY) mode (that only does instruction fetches). That is what
the separate vsyscall series would do once the LASS base is in.