KVM: nVMX: Treat General Detect #DB (DR7.GD=1) as fault-like

Exclude General Detect #DBs, which have fault-like behavior but also have
a non-zero payload (DR6.BD=1), from nVMX's handling of pending debug
traps.  Opportunistically rewrite the comment to better document what is
being checked, i.e. "has a non-zero payload" vs. "has a payload", and to
call out the many caveats surrounding #DBs that KVM dodges one way or
another.

Cc: Oliver Upton <oupton@google.com>
Cc: Peter Shier <pshier@google.com>
Fixes: 684c0422 ("KVM: nVMX: Handle pending #DB when injecting INIT VM-exit")
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Link: https://lore.kernel.org/r/20220830231614.3580124-7-seanjc@google.comSigned-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/nested.c

@@ -3899,16 +3899,29 @@ static void nested_vmx_inject_exception_vmexit(struct kvm_vcpu *vcpu,
 }
 
 /*
- * Returns true if a debug trap is pending delivery.
+ * Returns true if a debug trap is (likely) pending delivery.  Infer the class
+ * of a #DB (trap-like vs. fault-like) from the exception payload (to-be-DR6).
+ * Using the payload is flawed because code breakpoints (fault-like) and data
+ * breakpoints (trap-like) set the same bits in DR6 (breakpoint detected), i.e.
+ * this will return false positives if a to-be-injected code breakpoint #DB is
+ * pending (from KVM's perspective, but not "pending" across an instruction
+ * boundary).  ICEBP, a.k.a. INT1, is also not reflected here even though it
+ * too is trap-like.
  *
- * In KVM, debug traps bear an exception payload. As such, the class of a #DB
- * exception may be inferred from the presence of an exception payload.
+ * KVM "works" despite these flaws as ICEBP isn't currently supported by the
+ * emulator, Monitor Trap Flag is not marked pending on intercepted #DBs (the
+ * #DB has already happened), and MTF isn't marked pending on code breakpoints
+ * from the emulator (because such #DBs are fault-like and thus don't trigger
+ * actions that fire on instruction retire).
  */
-static inline bool vmx_pending_dbg_trap(struct kvm_vcpu *vcpu)
+static inline unsigned long vmx_get_pending_dbg_trap(struct kvm_vcpu *vcpu)
 {
-	return vcpu->arch.exception.pending &&
-			vcpu->arch.exception.nr == DB_VECTOR &&
-			vcpu->arch.exception.payload;
+	if (!vcpu->arch.exception.pending ||
+	    vcpu->arch.exception.nr != DB_VECTOR)
+		return 0;
+
+	/* General Detect #DBs are always fault-like. */
+	return vcpu->arch.exception.payload & ~DR6_BD;
 }
 
 /*
@@ -3920,9 +3933,10 @@ static inline bool vmx_pending_dbg_trap(struct kvm_vcpu *vcpu)
  */
 static void nested_vmx_update_pending_dbg(struct kvm_vcpu *vcpu)
 {
-	if (vmx_pending_dbg_trap(vcpu))
-		vmcs_writel(GUEST_PENDING_DBG_EXCEPTIONS,
-			    vcpu->arch.exception.payload);
+	unsigned long pending_dbg = vmx_get_pending_dbg_trap(vcpu);
+
+	if (pending_dbg)
+		vmcs_writel(GUEST_PENDING_DBG_EXCEPTIONS, pending_dbg);
 }
 
 static bool nested_vmx_preemption_timer_pending(struct kvm_vcpu *vcpu)
@@ -3979,7 +3993,7 @@ static int vmx_check_nested_events(struct kvm_vcpu *vcpu)
 	 * while delivering the pending exception.
 	 */
 
-	if (vcpu->arch.exception.pending && !vmx_pending_dbg_trap(vcpu)) {
+	if (vcpu->arch.exception.pending && !vmx_get_pending_dbg_trap(vcpu)) {
 		if (vmx->nested.nested_run_pending)
 			return -EBUSY;
 		if (!nested_vmx_check_exception(vcpu, &exit_qual))