linux-kernel - [PATCH 3/3] x86/cpu: Remove LASS restriction on EFI

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Message-ID: <20251204072143.3636863-4-sohil.mehta@intel.com>
Date: Wed,  3 Dec 2025 23:21:43 -0800
From: Sohil Mehta <sohil.mehta@...el.com>
To: 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>
Cc: "H . Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Kiryl Shutsemau <kas@...nel.org>,
	Sohil Mehta <sohil.mehta@...el.com>,
	Rick Edgecombe <rick.p.edgecombe@...el.com>,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	Tony Luck <tony.luck@...el.com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	linux-kernel@...r.kernel.org,
	linux-efi@...r.kernel.org
Subject: [PATCH 3/3] x86/cpu: Remove LASS restriction on EFI

The initial LASS enabling has been deferred to much later during boot,
and EFI runtime services now run with LASS temporarily disabled. This
removes LASS from the path of all EFI services.

To make LASS more usable, remove the restriction on EFI, as the two can
now coexist.

Signed-off-by: Sohil Mehta <sohil.mehta@...el.com>
---
 arch/x86/kernel/cpu/common.c | 7 +------
 1 file changed, 1 insertion(+), 6 deletions(-)

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index c6835a04d734..9c60084203b5 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -415,14 +415,9 @@ static __always_inline void setup_lass(struct cpuinfo_x86 *c)
 	 * Legacy vsyscall page access causes a #GP when LASS is active.
 	 * Disable LASS because the #GP handler doesn't support vsyscall
 	 * emulation.
-	 *
-	 * Also disable LASS when running under EFI, as some runtime and
-	 * boot services rely on 1:1 mappings in the lower half.
 	 */
-	if (IS_ENABLED(CONFIG_X86_VSYSCALL_EMULATION) ||
-	    IS_ENABLED(CONFIG_EFI)) {
+	if (IS_ENABLED(CONFIG_X86_VSYSCALL_EMULATION))
 		setup_clear_cpu_cap(X86_FEATURE_LASS);
-	}
 }
 
 static int enable_lass(unsigned int cpu)
-- 
2.43.0