[patch 05/24] x86/speculation: Disable STIBP when enhanced IBRS is in use

[Thomas Gleixner]

From: Tim Chen <tim.c.c...@linux.intel.com>

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

Disable STIBP when enhanced IBRS is used.

Signed-off-by: Tim Chen <tim.c.c...@linux.intel.com>
Signed-off-by: Thomas Gleixner <t...@linutronix.de>

---
 arch/x86/kernel/cpu/bugs.c |    7 +++++++
 1 file changed, 7 insertions(+)

--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -321,6 +321,10 @@ static bool stibp_needed(void)
        if (spectre_v2_enabled == SPECTRE_V2_NONE)
                return false;
 
+       /* Enhanced IBRS makes using STIBP unnecessary. */
+       if (spectre_v2_enabled == SPECTRE_V2_IBRS_ENHANCED)
+               return false;
+
        if (!boot_cpu_has(X86_FEATURE_STIBP))
                return false;
 
@@ -846,6 +850,9 @@ 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