If the L2 doesn't intercept interrupts, then the KVM will use vmcb02's
V_IRQ for L1 (to detect the interrupt window)
In this case on the nested VM exit KVM might need to copy the V_IRQ bit
from the vmcb02 to the vmcb01, to continue waiting for the
interrupt window.
To make it simple, just raise the KVM_REQ_EVENT request, which
execution will lead to the reenabling of the interrupt
window if needed.
Note that this is a theoretical bug because the KVM already does raise
the KVM_REQ_EVENT request one each nested VM exit because the nested
VM exit resets RFLAGS and the kvm_set_rflags() raises the
KVM_REQ_EVENT request in the response.
However raising this request explicitly, together with
documenting why this is needed, is still preferred.
Signed-off-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
---
arch/x86/kvm/svm/nested.c | 25 +++++++++++++++++++++++++
1 file changed, 25 insertions(+)
diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index aad3145b2f62fe..e891318595113e 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -1016,6 +1016,31 @@ int nested_svm_vmexit(struct vcpu_svm *svm)
svm_switch_vmcb(svm, &svm->vmcb01);
+ /* Note about synchronizing some of int_ctl bits from vmcb02 to vmcb01:
+ *
+ * - V_IRQ, V_IRQ_VECTOR, V_INTR_PRIO_MASK, V_IGN_TPR:
+ * If the L2 doesn't intercept interrupts, then
+ * (even if the L2 does use virtual interrupt masking),
+ * KVM will use the vmcb02's V_INTR to detect interrupt window.
+ *
+ * In this case, the KVM raises the KVM_REQ_EVENT to ensure that interrupt window
+ * is not lost and this implicitly copies these bits from vmcb02 to vmcb01
+ *
+ * V_TPR:
+ * If the L2 doesn't use virtual interrupt masking, then the L1's vTPR
+ * is stored in the vmcb02 but its value doesn't need to be copied from/to
+ * vmcb01 because it is copied from/to the TPR APIC's register on
+ * each VM entry/exit.
+ *
+ * V_GIF:
+ * - If the nested vGIF is not used, KVM uses vmcb02's V_GIF for L1's V_GIF,
+ * however, the L1 vGIF is reset to false on each VM exit, thus
+ * there is no need to copy it from vmcb02 to vmcb01.
+ */
+
+ if (!nested_exit_on_intr(svm))
+ kvm_make_request(KVM_REQ_EVENT, &svm->vcpu);
+
if (unlikely(svm->lbrv_enabled && (svm->nested.ctl.virt_ext & LBR_CTL_ENABLE_MASK))) {
svm_copy_lbrs(vmcb12, vmcb02);
svm_update_lbrv(vcpu);
--
2.26.3
