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

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Message-ID: <6C5D6437-5C95-41E1-BF88-0107C83B9CCB@zytor.com>
Date: Thu, 30 Oct 2025 23:41:54 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: David Laight <david.laight.linux@...il.com>,
        Sohil Mehta <sohil.mehta@...el.com>
CC: Andy Lutomirski <luto@...nel.org>,
        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 October 30, 2025 2:13:18 PM PDT, David Laight <david.laight.linux@...il.com> wrote:
>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
>

So I talked with Sohil about this earlier today, and there was a bit of a miscommunication — XONLY mode is just fine. It is read emulation mode that has problems.