Re: [PATCH v3 16/27] KVM: VMX: Virtualize FRED nested exception tracking

[Chao Gao]

>> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>> > index d81144bd648f..03f42b218554 100644
>> > --- a/arch/x86/kvm/vmx/vmx.c
>> > +++ b/arch/x86/kvm/vmx/vmx.c
>> > @@ -1910,8 +1910,11 @@ void vmx_inject_exception(struct kvm_vcpu *vcpu)
>> > 		vmcs_write32(VM_ENTRY_INSTRUCTION_LEN,
>> > 			     vmx->vcpu.arch.event_exit_inst_len);
>> > 		intr_info |= INTR_TYPE_SOFT_EXCEPTION;
>> > -	} else
>> > +	} else {
>> > 		intr_info |= INTR_TYPE_HARD_EXCEPTION;
>> > +		if (ex->nested)
>> > +			intr_info |= INTR_INFO_NESTED_EXCEPTION_MASK;
>> 
>> how about moving the is_fred_enable() check from kvm_multiple_exception() to here? i.e.,
>> 
>> 		if (ex->nested && is_fred_enabled(vcpu))
>> 			intr_info |= INTR_INFO_NESTED_EXCEPTION_MASK;
>> 
>> It is slightly clearer because FRED details don't bleed into kvm_multiple_exception().
>
>But FRED is all about events, including exception/interrupt/trap/...
>
>logically VMX nested exception only works when FRED is enabled, see how it is
>set at 2 places in kvm_multiple_exception().

"VMX nested exception only works ..." is what I referred to as "FRED details"

I believe there are several reasons to decouple the "nested exception" concept
from FRED:

1. Readers new to FRED can understand kvm_multiple_exception() without needing
   to know FRED details. Readers just need to know nested exceptions are
   exceptions encountered during delivering another event (exception/NMI/interrupts).

2. Developing KVM's generic "nested exception" concept can support other vendors.
   "nested" becomes a property of an exception. only how nested exceptions are
   reported to guests is specific to vendors (i.e., VMX/SVM).

3. This series handles ex->event_data in a similar approach: set it regardless
   of FRED enablement and let VMX/SVM code decide to consume or ignore it.