Subject: RE: [PATCH v1 13/23] KVM: VMX: Handle VMX nested exception for FRED
if (idt_vectoring_info &VECTORING_INFO_DELIVER_CODE_MASK)
kvm_requeue_exception_e(vcpu, vector,vmcs_read32(error_code_field),
idt_vectoring_info &INTR_INFO_NESTED_EXCEPTION_MASK);
elsekvm_requeue_exception(vcpu, vector,idt_vectoring_info &INTR_INFO_NESTED_EXCEPTION_MASK);
Exiting-event identification can also have bit 13 set, indicating a nested exception encountered and caused VM-exit. when reinjecting the exception to guests, kvm needs to set the "nested" bit, right? I suspect some changes to e.g., handle_exception_nmi() are needed.
The current patch relies on kvm_multiple_exception() to do that. But TBH, I'm not sure it can recognize all nested cases. I probably should revisit it.
So the conclusion is that kvm_multiple_exception() is smart enough, and a VMM doesn't have to check bit 13 of the Exiting-event identification.
In FRED spec 5.0, section 9.2 - New VMX Feature: VMX Nested-Exception Support, there is a statement at the end of Exiting-event identification:
(The value of this bit is always identical to that of the valid bit of the original-event identification field.)
It means that even w/o VMX Nested-Exception support, a VMM already knows if an exception is a nested exception encountered during delivery of another event in an exception caused VM exit (exit reason 0). This is done in KVM through reading IDT_VECTORING_INFO_FIELD and calling vmx_complete_interrupts() immediately after VM exits.
vmx_complete_interrupts() simply queues the original exception if there is one, and later the nested exception causing the VM exit could be cancelled if it is a shadow page fault. However if the shadow page fault is caused by a guest page fault, KVM injects it as a nested exception to have guest fix its page table.
I will add comments about this background in the next iteration.