[Patch v3 03/13] x86/speculation: Add static key for Enhanced IBRS

[Tim Chen]

Add static key to indicate whether we are using Enhanced IBRS to mitigate
Spectre v2.  This will be used in later patches to disengage STIBP code
for Spectre v2 mitigation as STIBP is not needed when Enhanced IBRS is
in use.

Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
---
 arch/x86/include/asm/nospec-branch.h | 3 +++
 arch/x86/kernel/cpu/bugs.c           | 4 ++++
 2 files changed, 7 insertions(+)

diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index fd2a8c1..d57e84e 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -3,6 +3,7 @@
 #ifndef _ASM_X86_NOSPEC_BRANCH_H_
 #define _ASM_X86_NOSPEC_BRANCH_H_
 
+#include <linux/static_key.h>
 #include <asm/alternative.h>
 #include <asm/alternative-asm.h>
 #include <asm/cpufeatures.h>
@@ -228,6 +229,8 @@ enum ssb_mitigation {
 extern char __indirect_thunk_start[];
 extern char __indirect_thunk_end[];
 
+DECLARE_STATIC_KEY_FALSE(spectre_v2_enhanced_ibrs);
+
 /*
  * On VMEXIT we must ensure that no RSB predictions learned in the guest
  * can be followed in the host, by overwriting the RSB completely. Both
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index b2f6b8b..2fc7b4e 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -142,6 +142,9 @@ static const char *spectre_v2_strings[] = {
 	[SPECTRE_V2_IBRS_ENHANCED]		= "Mitigation: Enhanced IBRS",
 };
 
+DEFINE_STATIC_KEY_FALSE(spectre_v2_enhanced_ibrs);
+EXPORT_SYMBOL(spectre_v2_enhanced_ibrs);
+
 #undef pr_fmt
 #define pr_fmt(fmt)     "Spectre V2 : " fmt
 
@@ -386,6 +389,7 @@ static void __init spectre_v2_select_mitigation(void)
 			/* Force it so VMEXIT will restore correctly */
 			x86_spec_ctrl_base |= SPEC_CTRL_IBRS;
 			wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
+			static_branch_enable(&spectre_v2_enhanced_ibrs);
 			goto specv2_set_mode;
 		}
 		if (IS_ENABLED(CONFIG_RETPOLINE))
-- 
2.9.4