KVM: nSVM: Disable intercept of VINTR if saved L1 host RFLAGS.IF is 0

Disable intercept of virtual interrupts (used to detect interrupt windows)
if the saved host (L1) RFLAGS.IF is '0', as the effective RFLAGS.IF for L1
interrupts will never be set while L2 is running (L2's RFLAGS.IF doesn't
affect L1 IRQs when virtual interrupts are enabled).

Suggested-by: Sean Christopherson <seanjc@google.com>
Link: https://lkml.kernel.org/r/Y9hybI65So5X2LFg%40google.com
Signed-off-by: Santosh Shukla <Santosh.Shukla@amd.com>
Link: https://lore.kernel.org/r/20230227084016.3368-3-santosh.shukla@amd.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sashukla1

arch/x86/kvm/svm/nested.c

@@ -139,13 +139,18 @@ void recalc_intercepts(struct vcpu_svm *svm)
 
 	if (g->int_ctl & V_INTR_MASKING_MASK) {
 		/*
-		 * Once running L2 with HF_VINTR_MASK, EFLAGS.IF and CR8
-		 * does not affect any interrupt we may want to inject;
-		 * therefore, writes to CR8 are irrelevant to L0, as are
-		 * interrupt window vmexits.
+		 * If L2 is active and V_INTR_MASKING is enabled in vmcb12,
+		 * disable intercept of CR8 writes as L2's CR8 does not affect
+		 * any interrupt KVM may want to inject.
+		 *
+		 * Similarly, disable intercept of virtual interrupts (used to
+		 * detect interrupt windows) if the saved RFLAGS.IF is '0', as
+		 * the effective RFLAGS.IF for L1 interrupts will never be set
+		 * while L2 is running (L2's RFLAGS.IF doesn't affect L1 IRQs).
 		 */
 		vmcb_clr_intercept(c, INTERCEPT_CR8_WRITE);
-		vmcb_clr_intercept(c, INTERCEPT_VINTR);
+		if (!(svm->vmcb01.ptr->save.rflags & X86_EFLAGS_IF))
+			vmcb_clr_intercept(c, INTERCEPT_VINTR);
 	}
 
 	/*

arch/x86/kvm/svm/svm.c

@@ -1583,6 +1583,16 @@ static void svm_set_vintr(struct vcpu_svm *svm)
 
 	svm_set_intercept(svm, INTERCEPT_VINTR);
 
+	/*
+	 * Recalculating intercepts may have cleared the VINTR intercept.  If
+	 * V_INTR_MASKING is enabled in vmcb12, then the effective RFLAGS.IF
+	 * for L1 physical interrupts is L1's RFLAGS.IF at the time of VMRUN.
+	 * Requesting an interrupt window if save.RFLAGS.IF=0 is pointless as
+	 * interrupts will never be unblocked while L2 is running.
+	 */
+	if (!svm_is_intercept(svm, INTERCEPT_VINTR))
+		return;
+
 	/*
 	 * This is just a dummy VINTR to actually cause a vmexit to happen.
 	 * Actual injection of virtual interrupts happens through EVENTINJ.