linux-kernel - Re: [PATCH v2 0/3] x86: Extend LASS support to EFI configurations

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Message-ID: <aYTXvX24FuBT49bd@agluck-desk3>
Date: Thu, 5 Feb 2026 09:47:41 -0800
From: "Luck, Tony" <tony.luck@...el.com>
To: Sohil Mehta <sohil.mehta@...el.com>
CC: <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>, Ard Biesheuvel <ardb@...nel.org>, "H . Peter Anvin"
	<hpa@...or.com>, Andy Lutomirski <luto@...nel.org>, Peter Zijlstra
	<peterz@...radead.org>, "Kirill A . Shutemov" <kas@...nel.org>, "Rick
 Edgecombe" <rick.p.edgecombe@...el.com>, Andrew Cooper
	<andrew.cooper3@...rix.com>, Alexander Shishkin
	<alexander.shishkin@...ux.intel.com>, <linux-kernel@...r.kernel.org>,
	<linux-efi@...r.kernel.org>
Subject: Re: [PATCH v2 0/3] x86: Extend LASS support to EFI configurations

On Tue, Jan 20, 2026 at 03:47:27PM -0800, Sohil Mehta wrote:
> Linear Address Space Separation (LASS) is currently disabled [1] when
> support for vsyscall emulation or EFI is compiled in. This series
> extends LASS support to EFI-enabled configurations.

I tried these patches on a LASS-capable system with a kernel built with
CONFIG_X86_VSYSCALL_EMULATION disabled, and traces added to efi_disable_lass()
and efi_enable_lass().

System boots OK (with a bunch of my trace messages showing that CR4.LASS
was disabled/enabled around EFI calls from several CPUs).

Just for grins I commented out the CR4 changes from the efi_disable_lass()
and efi_enable_lass(). System still boots OK, so it seems the EFI runtime
calls during boot don't have any of the BIOS bugs that require LASS disable.

Tested-by: Tony Luck <tony.luck@...el.com>

-Tony