[Patch v6 03/16] x86/speculation: Disable STIBP when enhanced IBRS is in use

[Tim Chen]

If enhanced IBRS is engaged, STIBP is redundant in mitigating Spectre
v2 user space exploits from hyperthread sibling.

Disable STIBP when enhanced IBRS is used.

Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
---
 arch/x86/kernel/cpu/bugs.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 3a6f13b..199f27e 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -323,11 +323,16 @@ static enum spectre_v2_mitigation_cmd __init spectre_v2_parse_cmdline(void)
 	return cmd;
 }
 
+/* Determine if STIBP should be always on. */
 static bool stibp_needed(void)
 {
 	if (spectre_v2_enabled == SPECTRE_V2_NONE)
 		return false;
 
+	/* Using enhanced IBRS makes using STIBP unnecessary. */
+	if (static_cpu_has(X86_FEATURE_USE_IBRS_ENHANCED))
+		return false;
+
 	if (!boot_cpu_has(X86_FEATURE_STIBP))
 		return false;
 
@@ -856,6 +861,9 @@ static ssize_t l1tf_show_state(char *buf)
 
 static char *stibp_state(void)
 {
+	if (spectre_v2_enabled == SPECTRE_V2_IBRS_ENHANCED)
+		return "";
+
 	if (x86_spec_ctrl_base & SPEC_CTRL_STIBP)
 		return ", STIBP";
 	else
-- 
2.9.4