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

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Message-ID: <50a8d23c-8c91-467c-9ee4-5894dc31d2f7@intel.com>
Date: Fri, 31 Oct 2025 10:21:30 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Sohil Mehta <sohil.mehta@...el.com>, 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>, "H . Peter Anvin" <hpa@...or.com>,
 Andy Lutomirski <luto@...nel.org>, Josh Poimboeuf <jpoimboe@...nel.org>,
 Peter Zijlstra <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 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@...r.kernel.org,
 linux-efi@...r.kernel.org
Subject: Re: [PATCH v11 9/9] x86/cpu: Enable LASS by default during CPU
 initialization

On 10/29/25 14:03, Sohil Mehta wrote:
...
> +static __always_inline void setup_lass(struct cpuinfo_x86 *c)
> +{
> +	if (cpu_feature_enabled(X86_FEATURE_LASS)) {
> +		/*
> +		 * Legacy vsyscall page access causes a #GP when LASS is
> +		 * active. However, vsyscall emulation isn't supported
> +		 * with #GP. To avoid breaking userspace, disable LASS
> +		 * if the emulation code is compiled in.
> +		 */
> +		if (IS_ENABLED(CONFIG_X86_VSYSCALL_EMULATION)) {
> +			pr_info_once("x86/cpu: Disabling LASS due to CONFIG_X86_VSYSCALL_EMULATION=y\n");
> +			setup_clear_cpu_cap(X86_FEATURE_LASS);
> +			return;
> +		}
> +
> +		cr4_set_bits(X86_CR4_LASS);
> +	}
> +}
This breaks two rules I have:

 1. Indent as little as practical
 2. Keep the main code flow at the lowest indentation level

IOW, this should be.

static __always_inline void setup_lass(struct cpuinfo_x86 *c)
{
	if (!cpu_feature_enabled(X86_FEATURE_LASS))
		return;
...

But I can fix that up when I apply this. I think it's mostly ready.

Reviewed-by: Dave Hansen <dave.hansen@...ux.intel.com>