[RFC][PATCH 08/17] x86/cpu: Add IBPB on VMEXIT to retbleed=

[Peter Zijlstra <peterz@infradead.org>]

Since IBPB-on-VMEXIT is an obvious variant of retbleed=ibpb, add it as
an such.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 arch/x86/kernel/cpu/bugs.c |   17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -751,6 +751,7 @@ enum retbleed_mitigation {
 	RETBLEED_MITIGATION_UNRET_SRSO,
 	RETBLEED_MITIGATION_UNRET_SRSO_ALIAS,
 	RETBLEED_MITIGATION_IBPB,
+	RETBLEED_MITIGATION_IBPB_VMEXIT,
 	RETBLEED_MITIGATION_IBRS,
 	RETBLEED_MITIGATION_EIBRS,
 	RETBLEED_MITIGATION_STUFF,
@@ -763,6 +764,7 @@ enum retbleed_mitigation_cmd {
 	RETBLEED_CMD_UNRET_SRSO,
 	RETBLEED_CMD_UNRET_SRSO_ALIAS,
 	RETBLEED_CMD_IBPB,
+	RETBLEED_CMD_IBPB_VMEXIT,
 	RETBLEED_CMD_STUFF,
 };
 
@@ -772,6 +774,7 @@ static const char * const retbleed_strin
 	[RETBLEED_MITIGATION_UNRET_SRSO]	= "Mitigation: srso untrained return thunk",
 	[RETBLEED_MITIGATION_UNRET_SRSO_ALIAS]	= "Mitigation: srso alias untrained return thunk",
 	[RETBLEED_MITIGATION_IBPB]		= "Mitigation: IBPB",
+	[RETBLEED_MITIGATION_IBPB_VMEXIT]	= "Mitigation: IBPB on VMEXIT only",
 	[RETBLEED_MITIGATION_IBRS]		= "Mitigation: IBRS",
 	[RETBLEED_MITIGATION_EIBRS]		= "Mitigation: Enhanced IBRS",
 	[RETBLEED_MITIGATION_STUFF]		= "Mitigation: Stuffing",
@@ -808,6 +811,8 @@ static int __init retbleed_parse_cmdline
 			retbleed_cmd = RETBLEED_CMD_UNRET_SRSO_ALIAS;
 		} else if (!strcmp(str, "ibpb")) {
 			retbleed_cmd = RETBLEED_CMD_IBPB;
+		} else if (!strcmp(str, "ibpb_vmexit")) {
+			retbleed_cmd = RETBLEED_CMD_IBPB_VMEXIT;
 		} else if (!strcmp(str, "stuff")) {
 			retbleed_cmd = RETBLEED_CMD_STUFF;
 		} else if (!strcmp(str, "nosmt")) {
@@ -881,13 +886,17 @@ static void __init retbleed_select_mitig
 		break;
 
 	case RETBLEED_CMD_IBPB:
+	case RETBLEED_CMD_IBPB_VMEXIT:
 		if (!boot_cpu_has(X86_FEATURE_IBPB)) {
 			pr_err("WARNING: CPU does not support IBPB.\n");
 			goto do_cmd_auto;
 		} else if (IS_ENABLED(CONFIG_CPU_IBPB_ENTRY)) {
 			if (boot_cpu_has_bug(X86_BUG_SRSO) && !has_microcode)
 				pr_err("IBPB-extending microcode not applied; SRSO NOT mitigated\n");
-			retbleed_mitigation = RETBLEED_MITIGATION_IBPB;
+			if (retbleed_cmd == RETBLEED_CMD_IBPB)
+				retbleed_mitigation = RETBLEED_MITIGATION_IBPB;
+			if (retbleed_cmd == RETBLEED_CMD_IBPB_VMEXIT)
+				retbleed_mitigation = RETBLEED_MITIGATION_IBPB_VMEXIT;
 		} else {
 			pr_err("WARNING: kernel not compiled with CPU_IBPB_ENTRY.\n");
 			goto do_cmd_auto;
@@ -961,6 +970,12 @@ static void __init retbleed_select_mitig
 
 	case RETBLEED_MITIGATION_IBPB:
 		setup_force_cpu_cap(X86_FEATURE_ENTRY_IBPB);
+		setup_force_cpu_cap(X86_FEATURE_IBPB_ON_VMEXIT);
+		mitigate_smt = true;
+		break;
+
+	case RETBLEED_MITIGATION_IBPB_VMEXIT:
+		setup_force_cpu_cap(X86_FEATURE_IBPB_ON_VMEXIT);
 		mitigate_smt = true;
 		break;