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>
Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com>
---
v2:
 - Picked up Dave's review tag
 - Improve commit message
---
 arch/x86/kernel/cpu/common.c | 15 ---------------
 1 file changed, 15 deletions(-)

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 3557f0e6b3aa..02472fc763d9 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -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)) {
-- 
2.43.0

On 2026-03-05 13:40, Sohil Mehta wrote:
> 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>
> Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com>
> ---
> v2:
>  - Picked up Dave's review tag
>  - Improve commit message
> ---
>  arch/x86/kernel/cpu/common.c | 15 ---------------
>  1 file changed, 15 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index 3557f0e6b3aa..02472fc763d9 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -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)) {

Love a patch that just deletes code!

Reviewed-by: H> Peter Anvin (Intel) <hpa@zytor.com>

The following commit has been merged into the x86/cpu branch of tip:

Commit-ID:     0e2a0cb1bf436a3979f3bf93e3b7fede5d93f730
Gitweb:        https://git.kernel.org/tip/0e2a0cb1bf436a3979f3bf93e3b7fede5d93f730
Author:        Sohil Mehta <sohil.mehta@intel.com>
AuthorDate:    Thu, 05 Mar 2026 13:40:26 -08:00
Committer:     Dave Hansen <dave.hansen@linux.intel.com>
CommitterDate: Thu, 05 Mar 2026 13:49:26 -08:00

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>
Link: https://patch.msgid.link/20260305214026.3887452-6-sohil.mehta@intel.com
---
 arch/x86/kernel/cpu/common.c | 15 ---------------
 1 file changed, 15 deletions(-)

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 3557f0e..02472fc 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -406,20 +406,6 @@ out:
 	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)) {