Commit 584d752b authored Mar 09, 2026 by Sohil Mehta Committed by Dave Hansen Mar 19, 2026

x86/cpu: Remove LASS restriction on vsyscall emulation



Vsyscall emulation has two modes of operation: XONLY and EMULATE. The
default XONLY mode is now supported with a LASS-triggered #GP. OTOH,
LASS is disabled if someone requests the deprecated EMULATE mode via the
vsyscall=emulate command line option. So, remove the restriction on LASS
when the overall vsyscall emulation support is compiled in.

As a result, there is no need for setup_lass() anymore. LASS is enabled
by default through a late_initcall().

Signed-off-by: Sohil Mehta <sohil.mehta@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Reviewed-by:
Tested-by: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
Link: https://patch.msgid.link/20260309181029.398498-6-sohil.mehta@intel.com

parent b36d1f53

arch/x86/kernel/cpu/common.c

+0 −15

Original line number	Diff line number	Diff line
		@@ -406,20 +406,6 @@ static __always_inline void setup_umip(struct cpuinfo_x86 *c)
		cr4_clear_bits(X86_CR4_UMIP);
		}

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

		/*
		* Legacy vsyscall page access causes a #GP when LASS is active.
		* Disable LASS because the #GP handler doesn't support vsyscall
		* emulation.
		*/
		if (IS_ENABLED(CONFIG_X86_VSYSCALL_EMULATION))
		setup_clear_cpu_cap(X86_FEATURE_LASS);
		}

		static int enable_lass(unsigned int cpu)
		{
		cr4_set_bits(X86_CR4_LASS);
		@@ -2061,7 +2047,6 @@ static void identify_cpu(struct cpuinfo_x86 *c)
		setup_smep(c);
		setup_smap(c);
		setup_umip(c);
		setup_lass(c);

		/* Enable FSGSBASE instructions if available. */
		if (cpu_has(c, X86_FEATURE_FSGSBASE)) {