[PATCH v2 2/2] x86/cpu, kvm: Support AMD Automatic IBRS

[Kim Phillips <kim.phillips@amd.com>]

The AMD Zen4 core supports a new feature called Automatic IBRS.

It is a "set-and-forget" feature that means that, like
Intel's Enhanced IBRS, h/w manages its IBRS mitigation
resources automatically across CPL transitions.

The feature is advertised by CPUID_Fn80000021_EAX bit 8 and is
enabled by setting MSR C000_0080 (EFER) bit 21.

Enable Automatic IBRS by default if the CPU feature is present.
It typically provides greater performance over the incumbent
generic retpolines mitigation.

Reuse the SPECTRE_V2_EIBRS spectre_v2_mitigation enum.
AMD Automatic IBRS and Intel Enhanced IBRS have similar
bugs.c enablement.

Also allow for spectre_v2=autoibrs on the kernel command line.
'spectre_v2=autoibrs,retpoline' and 'autoibrs,lfence' are
honoured but not required.  AutoIBRS will also be enabled if
the =eibrs[,{lfence,retpoline}] variants are specified.

Signed-off-by: Kim Phillips <kim.phillips@amd.com>
---
v1: https://lore.kernel.org/lkml/20221104213651.141057-3-kim.phillips@amd.com/
v2: Address v1 comments:
    - Reuse SPECTRE_V2_EIBRS spectre_v2_mitigation enum [Boris, PeterZ, D.Hansen]
      - Change from Boris' diff:
        Moved setting X86_FEATURE_IBRS_ENHANCED to after BUG_EIBRS_PBRSB
        so PBRSB mitigations wouldn't be enabled.
    - Allow for users to specify "autoibrs,lfence/retpoline" instead
      of actively preventing the extra protections.  AutoIBRS doesn't
      require the extra protection, but we allow it anyway.

 .../admin-guide/kernel-parameters.txt         |  9 +++++---
 arch/x86/include/asm/msr-index.h              |  2 ++
 arch/x86/kernel/cpu/bugs.c                    | 23 ++++++++++++-------
 arch/x86/kernel/cpu/common.c                  |  8 +++++++
 arch/x86/kvm/svm/svm.c                        |  3 +++
 arch/x86/kvm/x86.c                            |  3 +++
 6 files changed, 37 insertions(+), 11 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index a465d5242774..880016d06a8a 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5698,9 +5698,12 @@
 			retpoline,generic - Retpolines
 			retpoline,lfence  - LFENCE; indirect branch
 			retpoline,amd     - alias for retpoline,lfence
-			eibrs		  - enhanced IBRS
-			eibrs,retpoline   - enhanced IBRS + Retpolines
-			eibrs,lfence      - enhanced IBRS + LFENCE
+			eibrs		  - Enhanced/Auto IBRS
+			autoibrs	  - Enhanced/Auto IBRS
+			eibrs,retpoline   - Enhanced/Auto IBRS + Retpolines
+			autoibrs,retpoline- Enhanced/Auto IBRS + Retpolines
+			eibrs,lfence      - Enhanced/Auto IBRS + LFENCE
+			autoibrs,lfence   - Enhanced/Auto IBRS + LFENCE
 			ibrs		  - use IBRS to protect kernel
 
 			Not specifying this option is equivalent to
diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
index 8519191c6409..88fdd75f6a2f 100644
--- a/arch/x86/include/asm/msr-index.h
+++ b/arch/x86/include/asm/msr-index.h
@@ -30,6 +30,7 @@
 #define _EFER_SVME		12 /* Enable virtualization */
 #define _EFER_LMSLE		13 /* Long Mode Segment Limit Enable */
 #define _EFER_FFXSR		14 /* Enable Fast FXSAVE/FXRSTOR */
+#define _EFER_AUTOIBRS		21 /* Enable Automatic IBRS */
 
 #define EFER_SCE		(1<<_EFER_SCE)
 #define EFER_LME		(1<<_EFER_LME)
@@ -38,6 +39,7 @@
 #define EFER_SVME		(1<<_EFER_SVME)
 #define EFER_LMSLE		(1<<_EFER_LMSLE)
 #define EFER_FFXSR		(1<<_EFER_FFXSR)
+#define EFER_AUTOIBRS		(1<<_EFER_AUTOIBRS)
 
 /* Intel MSRs. Some also available on other CPUs */
 
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index aa0819252c88..5f48dd4dbc48 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1222,9 +1222,9 @@ static const char * const spectre_v2_strings[] = {
 	[SPECTRE_V2_NONE]			= "Vulnerable",
 	[SPECTRE_V2_RETPOLINE]			= "Mitigation: Retpolines",
 	[SPECTRE_V2_LFENCE]			= "Mitigation: LFENCE",
-	[SPECTRE_V2_EIBRS]			= "Mitigation: Enhanced IBRS",
-	[SPECTRE_V2_EIBRS_LFENCE]		= "Mitigation: Enhanced IBRS + LFENCE",
-	[SPECTRE_V2_EIBRS_RETPOLINE]		= "Mitigation: Enhanced IBRS + Retpolines",
+	[SPECTRE_V2_EIBRS]			= "Mitigation: Enhanced / Automatic IBRS",
+	[SPECTRE_V2_EIBRS_LFENCE]		= "Mitigation: Enhanced / Automatic IBRS + LFENCE",
+	[SPECTRE_V2_EIBRS_RETPOLINE]		= "Mitigation: Enhanced / Automatic IBRS + Retpolines",
 	[SPECTRE_V2_IBRS]			= "Mitigation: IBRS",
 };
 
@@ -1240,8 +1240,11 @@ static const struct {
 	{ "retpoline,lfence",	SPECTRE_V2_CMD_RETPOLINE_LFENCE,  false },
 	{ "retpoline,generic",	SPECTRE_V2_CMD_RETPOLINE_GENERIC, false },
 	{ "eibrs",		SPECTRE_V2_CMD_EIBRS,		  false },
+	{ "autoibrs",		SPECTRE_V2_CMD_EIBRS,		  false },
 	{ "eibrs,lfence",	SPECTRE_V2_CMD_EIBRS_LFENCE,	  false },
+	{ "autoibrs,lfence",	SPECTRE_V2_CMD_EIBRS_LFENCE,	  false },
 	{ "eibrs,retpoline",	SPECTRE_V2_CMD_EIBRS_RETPOLINE,	  false },
+	{ "autoibrs,retpoline",	SPECTRE_V2_CMD_EIBRS_RETPOLINE,	  false },
 	{ "auto",		SPECTRE_V2_CMD_AUTO,		  false },
 	{ "ibrs",		SPECTRE_V2_CMD_IBRS,              false },
 };
@@ -1293,7 +1296,7 @@ static enum spectre_v2_mitigation_cmd __init spectre_v2_parse_cmdline(void)
 	     cmd == SPECTRE_V2_CMD_EIBRS_LFENCE ||
 	     cmd == SPECTRE_V2_CMD_EIBRS_RETPOLINE) &&
 	    !boot_cpu_has(X86_FEATURE_IBRS_ENHANCED)) {
-		pr_err("%s selected but CPU doesn't have eIBRS. Switching to AUTO select\n",
+		pr_err("%s selected but CPU doesn't have Enhanced or Automatic IBRS. Switching to AUTO select\n",
 		       mitigation_options[i].option);
 		return SPECTRE_V2_CMD_AUTO;
 	}
@@ -1479,8 +1482,12 @@ static void __init spectre_v2_select_mitigation(void)
 		pr_err(SPECTRE_V2_EIBRS_EBPF_MSG);
 
 	if (spectre_v2_in_ibrs_mode(mode)) {
-		x86_spec_ctrl_base |= SPEC_CTRL_IBRS;
-		write_spec_ctrl_current(x86_spec_ctrl_base, true);
+		if (boot_cpu_has(X86_FEATURE_AUTOIBRS)) {
+			msr_set_bit(MSR_EFER, _EFER_AUTOIBRS);
+		} else {
+			x86_spec_ctrl_base |= SPEC_CTRL_IBRS;
+			write_spec_ctrl_current(x86_spec_ctrl_base, true);
+		}
 	}
 
 	switch (mode) {
@@ -1564,8 +1571,8 @@ static void __init spectre_v2_select_mitigation(void)
 	/*
 	 * Retpoline protects the kernel, but doesn't protect firmware.  IBRS
 	 * and Enhanced IBRS protect firmware too, so enable IBRS around
-	 * firmware calls only when IBRS / Enhanced IBRS aren't otherwise
-	 * enabled.
+	 * firmware calls only when IBRS / Enhanced / Automatic IBRS aren't
+	 * otherwise enabled.
 	 *
 	 * Use "mode" to check Enhanced IBRS instead of boot_cpu_has(), because
 	 * the user might select retpoline on the kernel command line and if
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 73cc546e024d..6506024f97e9 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1406,6 +1406,14 @@ static void __init cpu_set_bug_bits(struct cpuinfo_x86 *c)
 	    !(ia32_cap & ARCH_CAP_PBRSB_NO))
 		setup_force_cpu_bug(X86_BUG_EIBRS_PBRSB);
 
+	/*
+	 * AMD's AutoIBRS is equivalent to Intel's eIBRS - use the Intel flag only
+	 * after IBRS_ENHANCED bugs such as BUG_EIBRS_PBRSB above have been
+	 * determined.
+	 */
+	if (cpu_has(c, X86_FEATURE_AUTOIBRS))
+		setup_force_cpu_cap(X86_FEATURE_IBRS_ENHANCED);
+
 	if (cpu_matches(cpu_vuln_whitelist, NO_MELTDOWN))
 		return;
 
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 4b6d2b050e57..3ac3d4cfce24 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -4960,6 +4960,9 @@ static __init int svm_hardware_setup(void)
 
 	tsc_aux_uret_slot = kvm_add_user_return_msr(MSR_TSC_AUX);
 
+	if (boot_cpu_has(X86_FEATURE_AUTOIBRS))
+		kvm_enable_efer_bits(EFER_AUTOIBRS);
+
 	/* Check for pause filtering support */
 	if (!boot_cpu_has(X86_FEATURE_PAUSEFILTER)) {
 		pause_filter_count = 0;
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 490ec23c8450..db0f522fd597 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1682,6 +1682,9 @@ static int do_get_msr_feature(struct kvm_vcpu *vcpu, unsigned index, u64 *data)
 
 static bool __kvm_valid_efer(struct kvm_vcpu *vcpu, u64 efer)
 {
+	if (efer & EFER_AUTOIBRS && !guest_cpuid_has(vcpu, X86_FEATURE_AUTOIBRS))
+		return false;
+
 	if (efer & EFER_FFXSR && !guest_cpuid_has(vcpu, X86_FEATURE_FXSR_OPT))
 		return false;
 
-- 
2.34.1