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Subject: RE: [PATCH v1 13/23] KVM: VMX: Handle VMX nested exception for FRED
Thread-Topic: [PATCH v1 13/23] KVM: VMX: Handle VMX nested exception for FRED
Thread-Index: AQHaEnYXOQlvFLhpWkG4lt/jksLCm7B5dqYAgAFC3tCAIVOmcA==
Date:   Wed, 6 Dec 2023 08:37:39 +0000
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> Subject: RE: [PATCH v1 13/23] KVM: VMX: Handle VMX nested exception for F=
RED
>=20
> > >+		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);
> > >+		else
> > >+			kvm_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.
>=20
> 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.