x86/its: Add "vmexit" option to skip mitigation on some CPUs

Ice Lake generation CPUs are not affected by guest/host isolation part of
ITS. If a user is only concerned about KVM guests, they can now choose a
new cmdline option "vmexit" that will not deploy the ITS mitigation when
CPU is not affected by guest/host isolation. This saves the performance
overhead of ITS mitigation on Ice Lake gen CPUs.

When "vmexit" option selected, if the CPU is affected by ITS guest/host
isolation, the default ITS mitigation is deployed.

Signed-off-by: Pawan Gupta <[email protected]>
Signed-off-by: Dave Hansen <[email protected]>
Reviewed-by: Josh Poimboeuf <[email protected]>
Reviewed-by: Alexandre Chartre <[email protected]>

Author: pa1gupta

Documentation/admin-guide/kernel-parameters.txt

@@ -2210,6 +2210,8 @@
 			off:    Disable mitigation.
 			force:	Force the ITS bug and deploy default
 				mitigation.
+			vmexit: Only deploy mitigation if CPU is affected by
+				guest/host isolation part of ITS.
 
 			For details see:
 			Documentation/admin-guide/hw-vuln/indirect-target-selection.rst

arch/x86/include/asm/cpufeatures.h

@@ -535,4 +535,5 @@
 #define X86_BUG_IBPB_NO_RET	   	X86_BUG(1*32 + 4) /* "ibpb_no_ret" IBPB omits return target predictions */
 #define X86_BUG_SPECTRE_V2_USER		X86_BUG(1*32 + 5) /* "spectre_v2_user" CPU is affected by Spectre variant 2 attack between user processes */
 #define X86_BUG_ITS			X86_BUG(1*32 + 6) /* "its" CPU is affected by Indirect Target Selection */
+#define X86_BUG_ITS_NATIVE_ONLY		X86_BUG(1*32 + 7) /* "its_native_only" CPU is affected by ITS, VMX is not affected */
 #endif /* _ASM_X86_CPUFEATURES_H */

arch/x86/kernel/cpu/bugs.c

@@ -1203,16 +1203,19 @@ static void __init retbleed_select_mitigation(void)
 enum its_mitigation_cmd {
 	ITS_CMD_OFF,
 	ITS_CMD_ON,
+	ITS_CMD_VMEXIT,
 };
 
 enum its_mitigation {
 	ITS_MITIGATION_OFF,
+	ITS_MITIGATION_VMEXIT_ONLY,
 	ITS_MITIGATION_ALIGNED_THUNKS,
 	ITS_MITIGATION_RETPOLINE_STUFF,
 };
 
 static const char * const its_strings[] = {
 	[ITS_MITIGATION_OFF]			= "Vulnerable",
+	[ITS_MITIGATION_VMEXIT_ONLY]		= "Mitigation: Vulnerable, KVM: Not affected",
 	[ITS_MITIGATION_ALIGNED_THUNKS]		= "Mitigation: Aligned branch/return thunks",
 	[ITS_MITIGATION_RETPOLINE_STUFF]	= "Mitigation: Retpolines, Stuffing RSB",
 };
@@ -1239,6 +1242,8 @@ static int __init its_parse_cmdline(char *str)
 	} else if (!strcmp(str, "force")) {
 		its_cmd = ITS_CMD_ON;
 		setup_force_cpu_bug(X86_BUG_ITS);
+	} else if (!strcmp(str, "vmexit")) {
+		its_cmd = ITS_CMD_VMEXIT;
 	} else {
 		pr_err("Ignoring unknown indirect_target_selection option (%s).", str);
 	}
@@ -1294,6 +1299,12 @@ static void __init its_select_mitigation(void)
 	case ITS_CMD_OFF:
 		its_mitigation = ITS_MITIGATION_OFF;
 		break;
+	case ITS_CMD_VMEXIT:
+		if (boot_cpu_has_bug(X86_BUG_ITS_NATIVE_ONLY)) {
+			its_mitigation = ITS_MITIGATION_VMEXIT_ONLY;
+			goto out;
+		}
+		fallthrough;
 	case ITS_CMD_ON:
 		its_mitigation = ITS_MITIGATION_ALIGNED_THUNKS;
 		if (!boot_cpu_has(X86_FEATURE_RETPOLINE))

arch/x86/kernel/cpu/common.c

@@ -1229,6 +1229,8 @@ static const __initconst struct x86_cpu_id cpu_vuln_whitelist[] = {
 #define RFDS		BIT(7)
 /* CPU is affected by Indirect Target Selection */
 #define ITS		BIT(8)
+/* CPU is affected by Indirect Target Selection, but guest-host isolation is not affected */
+#define ITS_NATIVE_ONLY	BIT(9)
 
 static const struct x86_cpu_id cpu_vuln_blacklist[] __initconst = {
 	VULNBL_INTEL_STEPS(INTEL_IVYBRIDGE,	     X86_STEP_MAX,	SRBDS),
@@ -1249,16 +1251,16 @@ static const struct x86_cpu_id cpu_vuln_blacklist[] __initconst = {
 	VULNBL_INTEL_STEPS(INTEL_KABYLAKE,		      0xc,	MMIO | RETBLEED | GDS | SRBDS),
 	VULNBL_INTEL_STEPS(INTEL_KABYLAKE,	     X86_STEP_MAX,	MMIO | RETBLEED | GDS | SRBDS | ITS),
 	VULNBL_INTEL_STEPS(INTEL_CANNONLAKE_L,	     X86_STEP_MAX,	RETBLEED),
-	VULNBL_INTEL_STEPS(INTEL_ICELAKE_L,	     X86_STEP_MAX,	MMIO | MMIO_SBDS | RETBLEED | GDS | ITS),
-	VULNBL_INTEL_STEPS(INTEL_ICELAKE_D,	     X86_STEP_MAX,	MMIO | GDS | ITS),
-	VULNBL_INTEL_STEPS(INTEL_ICELAKE_X,	     X86_STEP_MAX,	MMIO | GDS | ITS),
+	VULNBL_INTEL_STEPS(INTEL_ICELAKE_L,	     X86_STEP_MAX,	MMIO | MMIO_SBDS | RETBLEED | GDS | ITS | ITS_NATIVE_ONLY),
+	VULNBL_INTEL_STEPS(INTEL_ICELAKE_D,	     X86_STEP_MAX,	MMIO | GDS | ITS | ITS_NATIVE_ONLY),
+	VULNBL_INTEL_STEPS(INTEL_ICELAKE_X,	     X86_STEP_MAX,	MMIO | GDS | ITS | ITS_NATIVE_ONLY),
 	VULNBL_INTEL_STEPS(INTEL_COMETLAKE,	     X86_STEP_MAX,	MMIO | MMIO_SBDS | RETBLEED | GDS | ITS),
 	VULNBL_INTEL_STEPS(INTEL_COMETLAKE_L,		      0x0,	MMIO | RETBLEED | ITS),
 	VULNBL_INTEL_STEPS(INTEL_COMETLAKE_L,	     X86_STEP_MAX,	MMIO | MMIO_SBDS | RETBLEED | GDS | ITS),
-	VULNBL_INTEL_STEPS(INTEL_TIGERLAKE_L,	     X86_STEP_MAX,	GDS | ITS),
-	VULNBL_INTEL_STEPS(INTEL_TIGERLAKE,	     X86_STEP_MAX,	GDS | ITS),
+	VULNBL_INTEL_STEPS(INTEL_TIGERLAKE_L,	     X86_STEP_MAX,	GDS | ITS | ITS_NATIVE_ONLY),
+	VULNBL_INTEL_STEPS(INTEL_TIGERLAKE,	     X86_STEP_MAX,	GDS | ITS | ITS_NATIVE_ONLY),
 	VULNBL_INTEL_STEPS(INTEL_LAKEFIELD,	     X86_STEP_MAX,	MMIO | MMIO_SBDS | RETBLEED),
-	VULNBL_INTEL_STEPS(INTEL_ROCKETLAKE,	     X86_STEP_MAX,	MMIO | RETBLEED | GDS | ITS),
+	VULNBL_INTEL_STEPS(INTEL_ROCKETLAKE,	     X86_STEP_MAX,	MMIO | RETBLEED | GDS | ITS | ITS_NATIVE_ONLY),
 	VULNBL_INTEL_TYPE(INTEL_ALDERLAKE,		     ATOM,	RFDS),
 	VULNBL_INTEL_STEPS(INTEL_ALDERLAKE_L,	     X86_STEP_MAX,	RFDS),
 	VULNBL_INTEL_TYPE(INTEL_RAPTORLAKE,		     ATOM,	RFDS),
@@ -1480,8 +1482,11 @@ static void __init cpu_set_bug_bits(struct cpuinfo_x86 *c)
 	if (cpu_has(c, X86_FEATURE_AMD_IBPB) && !cpu_has(c, X86_FEATURE_AMD_IBPB_RET))
 		setup_force_cpu_bug(X86_BUG_IBPB_NO_RET);
 
-	if (vulnerable_to_its(x86_arch_cap_msr))
+	if (vulnerable_to_its(x86_arch_cap_msr)) {
 		setup_force_cpu_bug(X86_BUG_ITS);
+		if (cpu_matches(cpu_vuln_blacklist, ITS_NATIVE_ONLY))
+			setup_force_cpu_bug(X86_BUG_ITS_NATIVE_ONLY);
+	}
 
 	if (cpu_matches(cpu_vuln_whitelist, NO_MELTDOWN))
 		return;