[RFC PATCH 5/9] x86/bugs: Use Virtual MSRs to request hardware mitigations

[Zhang Chen]

From: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>

Guests that have different family/model than the host may not be aware
of hardware mitigations(such as RRSBA_DIS_S) available on host. This is
particularly true when guests migrate. To solve this problem Intel
processors have added a virtual MSR interface through which guests can
report their mitigation status and request VMM to deploy relevant
hardware mitigations.

Use this virtualized MSR interface to request relevant hardware controls
for retpoline mitigation.

Signed-off-by: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
---
 arch/x86/include/asm/msr-index.h | 23 +++++++++++++++++++++++
 arch/x86/kernel/cpu/bugs.c       | 24 ++++++++++++++++++++++++
 2 files changed, 47 insertions(+)

diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
index 1143ac9400c3..1166b472377c 100644
--- a/arch/x86/include/asm/msr-index.h
+++ b/arch/x86/include/asm/msr-index.h
@@ -165,6 +165,7 @@
 						 * IA32_XAPIC_DISABLE_STATUS MSR
 						 * supported
 						 */
+#define ARCH_CAP_VIRTUAL_ENUM		BIT(63)	/* MSR_VIRTUAL_ENUMERATION supported */
 
 #define MSR_IA32_FLUSH_CMD		0x0000010b
 #define L1D_FLUSH			BIT(0)	/*
@@ -1062,6 +1063,28 @@
 #define MSR_IA32_VMX_MISC_INTEL_PT                 (1ULL << 14)
 #define MSR_IA32_VMX_MISC_VMWRITE_SHADOW_RO_FIELDS (1ULL << 29)
 #define MSR_IA32_VMX_MISC_PREEMPTION_TIMER_SCALE   0x1F
+
+/* Intel virtual MSRs */
+#define MSR_VIRTUAL_ENUMERATION			0x50000000
+#define VIRT_ENUM_MITIGATION_CTRL_SUPPORT	BIT(0)	/*
+							 * Mitigation ctrl via virtual
+							 * MSRs supported
+							 */
+
+#define MSR_VIRTUAL_MITIGATION_ENUM		0x50000001
+#define MITI_ENUM_BHB_CLEAR_SEQ_S_SUPPORT	BIT(0)	/* VMM supports BHI_DIS_S */
+#define MITI_ENUM_RETPOLINE_S_SUPPORT		BIT(1)	/* VMM supports RRSBA_DIS_S */
+
+#define MSR_VIRTUAL_MITIGATION_CTRL		0x50000002
+#define MITI_CTRL_BHB_CLEAR_SEQ_S_USED		BIT(0)	/*
+							 * Request VMM to deploy
+							 * BHI_DIS_S mitigation
+							 */
+#define MITI_CTRL_RETPOLINE_S_USED		BIT(1)	/*
+							 * Request VMM to deploy
+							 * RRSBA_DIS_S mitigation
+							 */
+
 /* AMD-V MSRs */
 
 #define MSR_VM_CR                       0xc0010114
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 3e3230cccaa7..a9e869f568ee 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1379,6 +1379,28 @@ static void __init spectre_v2_determine_rsb_fill_type_at_vmexit(enum spectre_v2_
 	dump_stack();
 }
 
+/* Speculation control using virtualized MSRs */
+static void __init spec_ctrl_setup_virtualized_msr(void)
+{
+	u64 msr_virt_enum, msr_mitigation_enum, msr_mitigation_ctrl;
+
+	if (!(x86_read_arch_cap_msr() & ARCH_CAP_VIRTUAL_ENUM))
+		return;
+
+	rdmsrl(MSR_VIRTUAL_ENUMERATION, msr_virt_enum);
+	if (!(msr_virt_enum & VIRT_ENUM_MITIGATION_CTRL_SUPPORT))
+		return;
+
+	rdmsrl(MSR_VIRTUAL_MITIGATION_ENUM, msr_mitigation_enum);
+	/* When retpoline is being used, request relevant hardware controls */
+	if (boot_cpu_has(X86_FEATURE_RETPOLINE) &&
+	    msr_mitigation_enum & MITI_ENUM_RETPOLINE_S_SUPPORT) {
+		rdmsrl(MSR_VIRTUAL_MITIGATION_CTRL, msr_mitigation_ctrl);
+		msr_mitigation_ctrl |= MITI_CTRL_RETPOLINE_S_USED;
+		wrmsrl(MSR_VIRTUAL_MITIGATION_CTRL, msr_mitigation_ctrl);
+	}
+}
+
 static void __init spectre_v2_select_mitigation(void)
 {
 	enum spectre_v2_mitigation_cmd cmd = spectre_v2_parse_cmdline();
@@ -1485,6 +1507,8 @@ static void __init spectre_v2_select_mitigation(void)
 	    mode == SPECTRE_V2_RETPOLINE)
 		spec_ctrl_disable_kernel_rrsba();
 
+	spec_ctrl_setup_virtualized_msr();
+
 	spectre_v2_enabled = mode;
 	pr_info("%s\n", spectre_v2_strings[mode]);
 
-- 
2.25.1