linux-kernel - Re: [PATCH v11 5/9] x86/efi: Disable LASS while mapping the EFI runtime services

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Message-ID: <3e9c4fdd-88a8-4597-9405-d865fb837d95@intel.com>
Date: Fri, 31 Oct 2025 10:11:40 -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 5/9] x86/efi: Disable LASS while mapping the EFI
 runtime services

On 10/29/25 14:03, Sohil Mehta wrote:
> From: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
> 
> While mapping EFI runtime services, set_virtual_address_map() is called
> at its lower mapping, which LASS prohibits. Wrapping the EFI call with
> lass_disable()/_enable() is not enough, because the AC flag only
> controls data accesses, and not instruction fetches.
> 
> Use the big hammer and toggle the CR4.LASS bit to make this work.

One thing that's actually missing here is an explanation on how it's OK
to munge CR bits here. Why are preemption and interrupts not a problem?

A reviewer would have to go off and figure this out on their own.