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

[Lendacky, Thomas]

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. To show
that this transition has occurred, create a new spectre_v2_user_mitigation
value and a new spectre_v2_user_strings message. The new mitigation value
is used in spectre_v2_user_select_mitigation() to print the new mitigation
message as well as to return a new string from stibp_state().

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

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

 arch/x86/include/asm/cpufeatures.h   |    1 +
 arch/x86/include/asm/nospec-branch.h |    1 +
 arch/x86/kernel/cpu/bugs.c           |   28 ++++++++++++++++++++++------
 3 files changed, 24 insertions(+), 6 deletions(-)

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/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 032b600..dad12b7 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -232,6 +232,7 @@ enum spectre_v2_mitigation {
 enum spectre_v2_user_mitigation {
 	SPECTRE_V2_USER_NONE,
 	SPECTRE_V2_USER_STRICT,
+	SPECTRE_V2_USER_STRICT_PREFERRED,
 	SPECTRE_V2_USER_PRCTL,
 	SPECTRE_V2_USER_SECCOMP,
 };
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 58689ac..e53cefa 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -262,10 +262,11 @@ enum spectre_v2_user_cmd {
 };
 
 static const char * const spectre_v2_user_strings[] = {
-	[SPECTRE_V2_USER_NONE]		= "User space: Vulnerable",
-	[SPECTRE_V2_USER_STRICT]	= "User space: Mitigation: STIBP protection",
-	[SPECTRE_V2_USER_PRCTL]		= "User space: Mitigation: STIBP via prctl",
-	[SPECTRE_V2_USER_SECCOMP]	= "User space: Mitigation: STIBP via seccomp and prctl",
+	[SPECTRE_V2_USER_NONE]			= "User space: Vulnerable",
+	[SPECTRE_V2_USER_STRICT]		= "User space: Mitigation: STIBP protection",
+	[SPECTRE_V2_USER_STRICT_PREFERRED]	= "User space: Mitigation: STIBP always-on preferred protection",
+	[SPECTRE_V2_USER_PRCTL]			= "User space: Mitigation: STIBP via prctl",
+	[SPECTRE_V2_USER_SECCOMP]		= "User space: Mitigation: STIBP via seccomp and prctl",
 };
 
 static const struct {
@@ -355,6 +356,15 @@ 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))
+		mode = SPECTRE_V2_USER_STRICT_PREFERRED;
+
 	/* Initialize Indirect Branch Prediction Barrier */
 	if (boot_cpu_has(X86_FEATURE_IBPB)) {
 		setup_force_cpu_cap(X86_FEATURE_USE_IBPB);
@@ -610,6 +620,7 @@ void arch_smt_update(void)
 	case SPECTRE_V2_USER_NONE:
 		break;
 	case SPECTRE_V2_USER_STRICT:
+	case SPECTRE_V2_USER_STRICT_PREFERRED:
 		update_stibp_strict();
 		break;
 	case SPECTRE_V2_USER_PRCTL:
@@ -812,7 +823,8 @@ static int ib_prctl_set(struct task_struct *task, unsigned long ctrl)
 		 * Indirect branch speculation is always disabled in strict
 		 * mode.
 		 */
-		if (spectre_v2_user == SPECTRE_V2_USER_STRICT)
+		if (spectre_v2_user == SPECTRE_V2_USER_STRICT ||
+		    spectre_v2_user == SPECTRE_V2_USER_STRICT_PREFERRED)
 			return -EPERM;
 		task_clear_spec_ib_disable(task);
 		task_update_spec_tif(task);
@@ -825,7 +837,8 @@ static int ib_prctl_set(struct task_struct *task, unsigned long ctrl)
 		 */
 		if (spectre_v2_user == SPECTRE_V2_USER_NONE)
 			return -EPERM;
-		if (spectre_v2_user == SPECTRE_V2_USER_STRICT)
+		if (spectre_v2_user == SPECTRE_V2_USER_STRICT ||
+		    spectre_v2_user == SPECTRE_V2_USER_STRICT_PREFERRED)
 			return 0;
 		task_set_spec_ib_disable(task);
 		if (ctrl == PR_SPEC_FORCE_DISABLE)
@@ -896,6 +909,7 @@ static int ib_prctl_get(struct task_struct *task)
 			return PR_SPEC_PRCTL | PR_SPEC_DISABLE;
 		return PR_SPEC_PRCTL | PR_SPEC_ENABLE;
 	case SPECTRE_V2_USER_STRICT:
+	case SPECTRE_V2_USER_STRICT_PREFERRED:
 		return PR_SPEC_DISABLE;
 	default:
 		return PR_SPEC_NOT_AFFECTED;
@@ -1089,6 +1103,8 @@ static char *stibp_state(void)
 		return ", STIBP: disabled";
 	case SPECTRE_V2_USER_STRICT:
 		return ", STIBP: forced";
+	case SPECTRE_V2_USER_STRICT_PREFERRED:
+		return ", STIBP: always-on preferred";
 	case SPECTRE_V2_USER_PRCTL:
 	case SPECTRE_V2_USER_SECCOMP:
 		if (static_key_enabled(&switch_to_cond_stibp))