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

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Message-Id: <202682ea-304d-43b2-8a69-c8bc9d8e3e2c@app.fastmail.com>
Date: Thu, 30 Oct 2025 10:31:56 -0700
From: "Andy Lutomirski" <luto@...nel.org>
To: "Sohil Mehta" <sohil.mehta@...el.com>, "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 Thu, Oct 30, 2025, at 10:24 AM, Sohil Mehta wrote:
> 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.

Ah, I misunderstood you. I agree.