[PATCH 6/7] x86/svm: Set IBPB when running a different VCPU

[Paolo Bonzini]

From: Tom Lendacky <thomas.lendacky@xxxxxxx>

Set IBPB (Indirect Branch Prediction Barrier) when the current CPU is
going to run a VCPU different from what was previously run.  Nested
virtualization uses the same VMCB for the second level guest, but the
L1 hypervisor should be using IBRS to protect itself.

Signed-off-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
---
 arch/x86/kvm/svm.c | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 779981a00d01..dd744d896cec 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -289,6 +289,7 @@ struct amd_svm_iommu_ir {
 module_param(vgif, int, 0444);
  
 static bool __read_mostly have_spec_ctrl;
+static bool __read_mostly have_ibpb_support;
 
 static void svm_set_cr0(struct kvm_vcpu *vcpu, unsigned long cr0);
 static void svm_flush_tlb(struct kvm_vcpu *vcpu, bool invalidate_gpa);
@@ -540,6 +541,7 @@ struct svm_cpu_data {
 	struct kvm_ldttss_desc *tss_desc;
 
 	struct page *save_area;
+	struct vmcb *current_vmcb;
 };
 
 static DEFINE_PER_CPU(struct svm_cpu_data *, svm_data);
@@ -1151,6 +1153,11 @@ static __init int svm_hardware_setup(void)
 		pr_info("kvm: SPEC_CTRL available\n");
 	else
 		pr_info("kvm: SPEC_CTRL not available\n");
+	have_ibpb_support = have_spec_ctrl || cpu_has_ibpb_support();
+	if (have_ibpb_support)
+		pr_info("kvm: IBPB_SUPPORT available\n");
+	else
+		pr_info("kvm: IBPB_SUPPORT not available\n");
 
 	return 0;
 
@@ -1725,11 +1732,19 @@ static void svm_free_vcpu(struct kvm_vcpu *vcpu)
 	__free_pages(virt_to_page(svm->nested.msrpm), MSRPM_ALLOC_ORDER);
 	kvm_vcpu_uninit(vcpu);
 	kmem_cache_free(kvm_vcpu_cache, svm);
+
+	/*
+	 * The VMCB could be recycled, causing a false negative in
+	 * svm_vcpu_load; block speculative execution.
+	 */
+	if (have_ibpb_support)
+		wrmsrl(MSR_IA32_PRED_CMD, FEATURE_SET_IBPB);
 }
 
 static void svm_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 {
 	struct vcpu_svm *svm = to_svm(vcpu);
+	struct svm_cpu_data *sd = per_cpu(svm_data, cpu);
 	int i;
 
 	if (unlikely(cpu != vcpu->cpu)) {
@@ -1758,6 +1773,12 @@ static void svm_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 	if (static_cpu_has(X86_FEATURE_RDTSCP))
 		wrmsrl(MSR_TSC_AUX, svm->tsc_aux);
 
+	if (sd->current_vmcb != svm->vmcb) {
+		sd->current_vmcb = svm->vmcb;
+		if (have_ibpb_support)
+			wrmsrl(MSR_IA32_PRED_CMD, FEATURE_SET_IBPB);
+	}
+
 	avic_vcpu_load(vcpu, cpu);
 }
 
@@ -2798,6 +2819,11 @@ static int nested_svm_vmexit(struct vcpu_svm *svm)
 	if (!nested_vmcb)
 		return 1;
 
+	/*
+	 * No need for IBPB here, the L1 hypervisor should be running with
+	 * IBRS=1 and inserts one already when switching L2 VMs.
+	 */
+
 	/* Exit Guest-Mode */
 	leave_guest_mode(&svm->vcpu);
 	svm->nested.vmcb = 0;
@@ -3061,6 +3087,11 @@ static bool nested_svm_vmrun(struct vcpu_svm *svm)
 	if (!nested_vmcb)
 		return false;
 
+	/*
+	 * No need for IBPB here, since the nested VM is less privileged.  The
+	 * L1 hypervisor inserts one already when switching L2 VMs.
+	 */
+
 	if (!nested_vmcb_checks(nested_vmcb)) {
 		nested_vmcb->control.exit_code    = SVM_EXIT_ERR;
 		nested_vmcb->control.exit_code_hi = 0;
-- 
1.8.3.1