[PATCH v2] x86/speculation: Add support for STIBP always-on preferred mode

["Lendacky, Thomas" <Thomas.Lendacky@amd.com>]

Different AMD processors may have different implementations of STIBP.
When STIBP is conditionally enabled, some implementations would benefit
from having STIBP always on instead of toggling the STIBP bit through MSR
writes. This preference is advertised through a CPUID feature bit.

When conditional STIBP support is requested at boot and the CPU advertises
STIBP always-on mode as preferred, switch to STIBP "on" support. Print a
message to let the user know this occurred. Also, provide a boolean that
be used in stibp_state() to return a message tailored to the always-on
support.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
---

This patch is against the x86/pti branch of the tip tree:
  git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git x86/pti

Changes from v1:
- Removed explicit SPECTRE_V2_USER_STRICT_PREFERRED mode
- Added a message when switching to always-on mode
- Set and used a static boolean for the string in stibp_state()

 arch/x86/include/asm/cpufeatures.h |    1 +
 arch/x86/kernel/cpu/bugs.c         |   17 ++++++++++++++++-
 2 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index 28c4a50..df8e94e2 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -284,6 +284,7 @@
 #define X86_FEATURE_AMD_IBPB		(13*32+12) /* "" Indirect Branch Prediction Barrier */
 #define X86_FEATURE_AMD_IBRS		(13*32+14) /* "" Indirect Branch Restricted Speculation */
 #define X86_FEATURE_AMD_STIBP		(13*32+15) /* "" Single Thread Indirect Branch Predictors */
+#define X86_FEATURE_AMD_STIBP_ALWAYS_ON	(13*32+17) /* "" Single Thread Indirect Branch Predictors always-on preferred */
 #define X86_FEATURE_AMD_SSBD		(13*32+24) /* "" Speculative Store Bypass Disable */
 #define X86_FEATURE_VIRT_SSBD		(13*32+25) /* Virtualized Speculative Store Bypass Disable */
 #define X86_FEATURE_AMD_SSB_NO		(13*32+26) /* "" Speculative Store Bypass is fixed in hardware. */
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 58689ac..db156e1 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -61,6 +61,8 @@
 /* Control unconditional IBPB in switch_mm() */
 DEFINE_STATIC_KEY_FALSE(switch_mm_always_ibpb);
 
+static bool stibp_always_on;
+
 void __init check_bugs(void)
 {
 	identify_boot_cpu();
@@ -355,6 +357,18 @@ static void __init spec_v2_user_print_cond(const char *reason, bool secure)
 		break;
 	}
 
+	/*
+	 * At this point, an STIBP mode other than "off" has been set.
+	 * If STIBP support is not being forced, check if STIBP always-on
+	 * is preferred.
+	 */
+	if (mode != SPECTRE_V2_USER_STRICT &&
+	    boot_cpu_has(X86_FEATURE_AMD_STIBP_ALWAYS_ON)) {
+		stibp_always_on = true;
+		mode = SPECTRE_V2_USER_STRICT;
+		pr_info("mitigation: STIBP always-on is preferred\n");
+	}
+
 	/* Initialize Indirect Branch Prediction Barrier */
 	if (boot_cpu_has(X86_FEATURE_IBPB)) {
 		setup_force_cpu_cap(X86_FEATURE_USE_IBPB);
@@ -1088,7 +1102,8 @@ static char *stibp_state(void)
 	case SPECTRE_V2_USER_NONE:
 		return ", STIBP: disabled";
 	case SPECTRE_V2_USER_STRICT:
-		return ", STIBP: forced";
+		return stibp_always_on ? ", STIBP: always-on"
+				       : ", STIBP: forced";
 	case SPECTRE_V2_USER_PRCTL:
 	case SPECTRE_V2_USER_SECCOMP:
 		if (static_key_enabled(&switch_to_cond_stibp))