[PATCH 5/7] x86/cpu: Enable LASS (Linear Address Space Separation)

[Yian Chen]

LASS is enabled via setting a CR4 bit if the platform
supports the feature.

LASS may be disabled in early boot time, for example,
by command line parameter clearcpuid=lass/390 or
vsyscall flag. In such cases, the CPU feature and CR4
bits will be cleared.

Signed-off-by: Yian Chen <yian.chen@xxxxxxxxx>
Reviewed-by: Tony Luck <tony.luck@xxxxxxxxx>
---
 arch/x86/kernel/cpu/common.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 9cfca3d7d0e2..efc7c7623968 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -412,6 +412,23 @@ 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)) {
+		cr4_set_bits(X86_CR4_LASS);
+	} else {
+		/*
+		 * only clear the feature and cr4 bits when hardware
+		 * supports LASS, in case it was enabled in a previous
+		 * boot (e.g., via kexec)
+		 */
+		if (cpu_has(c, X86_FEATURE_LASS)) {
+			cr4_clear_bits(X86_CR4_LASS);
+			clear_cpu_cap(c, X86_FEATURE_LASS);
+		}
+	}
+}
+
 /* These bits should not change their value after CPU init is finished. */
 static const unsigned long cr4_pinned_mask =
 	X86_CR4_SMEP | X86_CR4_SMAP | X86_CR4_UMIP |
@@ -1848,6 +1865,7 @@ 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)) {
-- 
2.34.1