KVM: x86: Clean up and document nested #PF workaround

Replace the per-vendor hack-a-fix for KVM's #PF => #PF => #DF workaround
with an explicit, common workaround in kvm_inject_emulated_page_fault().
Aside from being a hack, the current approach is brittle and incomplete,
e.g. nSVM's KVM_SET_NESTED_STATE fails to set ->inject_page_fault(),
and nVMX fails to apply the workaround when VMX is intercepting #PF due
to allow_smaller_maxphyaddr=1.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index e1c695f..e46bd28 100644 (file)
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1516,6 +1516,8 @@ struct kvm_x86_ops {
 struct kvm_x86_nested_ops {
        void (*leave_nested)(struct kvm_vcpu *vcpu);
        int (*check_events)(struct kvm_vcpu *vcpu);
+       bool (*handle_page_fault_workaround)(struct kvm_vcpu *vcpu,
+                                            struct x86_exception *fault);
        bool (*hv_timer_pending)(struct kvm_vcpu *vcpu);
        void (*triple_fault)(struct kvm_vcpu *vcpu);
        int (*get_state)(struct kvm_vcpu *vcpu,

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index caa6912..bed5e16 100644 (file)
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -55,24 +55,26 @@ static void nested_svm_inject_npf_exit(struct kvm_vcpu *vcpu,
        nested_svm_vmexit(svm);
 }
 
-static void svm_inject_page_fault_nested(struct kvm_vcpu *vcpu, struct x86_exception *fault)
+static bool nested_svm_handle_page_fault_workaround(struct kvm_vcpu *vcpu,
+                                                   struct x86_exception *fault)
 {
        struct vcpu_svm *svm = to_svm(vcpu);
        struct vmcb *vmcb = svm->vmcb;
 
-       WARN_ON(!is_guest_mode(vcpu));
+       WARN_ON(!is_guest_mode(vcpu));
 
        if (vmcb12_is_intercept(&svm->nested.ctl,
                                INTERCEPT_EXCEPTION_OFFSET + PF_VECTOR) &&
-                               !svm->nested.nested_run_pending) {
-               vmcb->control.exit_code = SVM_EXIT_EXCP_BASE + PF_VECTOR;
+           !WARN_ON_ONCE(svm->nested.nested_run_pending)) {
+               vmcb->control.exit_code = SVM_EXIT_EXCP_BASE + PF_VECTOR;
                vmcb->control.exit_code_hi = 0;
                vmcb->control.exit_info_1 = fault->error_code;
                vmcb->control.exit_info_2 = fault->address;
                nested_svm_vmexit(svm);
-       } else {
-               kvm_inject_page_fault(vcpu, fault);
+               return true;
        }
+
+       return false;
 }
 
 static u64 nested_svm_get_tdp_pdptr(struct kvm_vcpu *vcpu, int index)
@@ -751,9 +753,6 @@ int enter_svm_guest_mode(struct kvm_vcpu *vcpu, u64 vmcb12_gpa,
        if (ret)
                return ret;
 
-       if (!npt_enabled)
-               vcpu->arch.mmu->inject_page_fault = svm_inject_page_fault_nested;
-
        if (!from_vmrun)
                kvm_make_request(KVM_REQ_GET_NESTED_STATE_PAGES, vcpu);
 
@@ -1659,6 +1658,7 @@ static bool svm_get_nested_state_pages(struct kvm_vcpu *vcpu)
 struct kvm_x86_nested_ops svm_nested_ops = {
        .leave_nested = svm_leave_nested,
        .check_events = svm_check_nested_events,
+       .handle_page_fault_workaround = nested_svm_handle_page_fault_workaround,
        .triple_fault = nested_svm_triple_fault,
        .get_nested_state_pages = svm_get_nested_state_pages,
        .get_state = svm_get_nested_state,

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 838ac7a..a668866 100644 (file)
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -476,24 +476,23 @@ static int nested_vmx_check_exception(struct kvm_vcpu *vcpu, unsigned long *exit
        return 0;
 }
 
-
-static void vmx_inject_page_fault_nested(struct kvm_vcpu *vcpu,
-               struct x86_exception *fault)
+static bool nested_vmx_handle_page_fault_workaround(struct kvm_vcpu *vcpu,
+                                                   struct x86_exception *fault)
 {
        struct vmcs12 *vmcs12 = get_vmcs12(vcpu);
 
        WARN_ON(!is_guest_mode(vcpu));
 
        if (nested_vmx_is_page_fault_vmexit(vmcs12, fault->error_code) &&
-               !to_vmx(vcpu)->nested.nested_run_pending) {
+           !WARN_ON_ONCE(to_vmx(vcpu)->nested.nested_run_pending)) {
                vmcs12->vm_exit_intr_error_code = fault->error_code;
                nested_vmx_vmexit(vcpu, EXIT_REASON_EXCEPTION_NMI,
                                  PF_VECTOR | INTR_TYPE_HARD_EXCEPTION |
                                  INTR_INFO_DELIVER_CODE_MASK | INTR_INFO_VALID_MASK,
                                  fault->address);
-       } else {
-               kvm_inject_page_fault(vcpu, fault);
+               return true;
        }
+       return false;
 }
 
 static int nested_vmx_check_io_bitmap_controls(struct kvm_vcpu *vcpu,
@@ -2614,9 +2613,6 @@ static int prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12,
                vmcs_write64(GUEST_PDPTR3, vmcs12->guest_pdptr3);
        }
 
-       if (!enable_ept)
-               vcpu->arch.walk_mmu->inject_page_fault = vmx_inject_page_fault_nested;
-
        if ((vmcs12->vm_entry_controls & VM_ENTRY_LOAD_IA32_PERF_GLOBAL_CTRL) &&
            WARN_ON_ONCE(kvm_set_msr(vcpu, MSR_CORE_PERF_GLOBAL_CTRL,
                                     vmcs12->guest_ia32_perf_global_ctrl))) {
@@ -6830,6 +6826,7 @@ __init int nested_vmx_hardware_setup(int (*exit_handlers[])(struct kvm_vcpu *))
 struct kvm_x86_nested_ops vmx_nested_ops = {
        .leave_nested = vmx_leave_nested,
        .check_events = vmx_check_nested_events,
+       .handle_page_fault_workaround = nested_vmx_handle_page_fault_workaround,
        .hv_timer_pending = nested_vmx_preemption_timer_pending,
        .triple_fault = nested_vmx_triple_fault,
        .get_state = vmx_get_nested_state,

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 1bc6799..b2b1772 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -748,6 +748,7 @@ void kvm_inject_page_fault(struct kvm_vcpu *vcpu, struct x86_exception *fault)
 }
 EXPORT_SYMBOL_GPL(kvm_inject_page_fault);
 
+/* Returns true if the page fault was immediately morphed into a VM-Exit. */
 bool kvm_inject_emulated_page_fault(struct kvm_vcpu *vcpu,
                                    struct x86_exception *fault)
 {
@@ -766,8 +767,26 @@ bool kvm_inject_emulated_page_fault(struct kvm_vcpu *vcpu,
                kvm_mmu_invalidate_gva(vcpu, fault_mmu, fault->address,
                                       fault_mmu->root.hpa);
 
+       /*
+        * A workaround for KVM's bad exception handling.  If KVM injected an
+        * exception into L2, and L2 encountered a #PF while vectoring the
+        * injected exception, manually check to see if L1 wants to intercept
+        * #PF, otherwise queuing the #PF will lead to #DF or a lost exception.
+        * In all other cases, defer the check to nested_ops->check_events(),
+        * which will correctly handle priority (this does not).  Note, other
+        * exceptions, e.g. #GP, are theoretically affected, #PF is simply the
+        * most problematic, e.g. when L0 and L1 are both intercepting #PF for
+        * shadow paging.
+        *
+        * TODO: Rewrite exception handling to track injected and pending
+        *       (VM-Exit) exceptions separately.
+        */
+       if (unlikely(vcpu->arch.exception.injected && is_guest_mode(vcpu)) &&
+           kvm_x86_ops.nested_ops->handle_page_fault_workaround(vcpu, fault))
+               return true;
+
        fault_mmu->inject_page_fault(vcpu, fault);
-       return fault->nested_page_fault;
+       return false;
 }
 EXPORT_SYMBOL_GPL(kvm_inject_emulated_page_fault);