RE: [RESEND PATCH 5/6] KVM: x86/VMX: add kvm_vmx_reinject_nmi_irq() for NMI/IRQ reinjection

Li, Xin3
2022-11-12T06:35:49Z

> > > So instead of fixing it, they made it worse :-(
> > >
> > > And now FRED is arguably making it worse again, and people wonder
> > > why I hate virt...
> >
> > Maybe I take it wrong, but FRED doesn't make anything worse. Fred
> > entry code will call external_interrupt() immediately for IRQs.
>
> But what about NMIs, afaict this is all horribly broken for NMIs.

NMIs are NOT handled by external_interrupt(), which is introduced in patch 4.

The NMI handling is added to kvm_vmx_reinject_nmi_irq() in patch 5 purely for VMX only.

>
> So the whole VMX thing latches the NMI (which stops NMI recursion), right?
>
> But then you drop out of noinstr code, which means any random exception can
> happen (kprobes #BP, hw_breakpoint #DB, or even #PF due to random
> nonsense like *SAN). This exception will do IRET and clear the NMI latch, all
> before you get to run any of the NMI code.
>
> Note how the normal NMI code is very careful to clear DR7 and both kprobes
> and hw_breakpoint know not to accept noinstr code as targets.
>
> You threw all that out the window.
>
> Also, NMI is IST, and with FRED it will run on a different stack as well, directly
> calling external_interrupt() doesn't honour that either.
>
> > You really really don't like the context how VMX dispatches NMI/IRQs
> > (which has been there for a long time), right?
>
> I really really hate this with a passion. The fact that it's been this way is no
> justification for keeping it. Crap is crap.
>
> Intel should have taken an example of SVM in this regard, and not doubled
> down and extended this NMI hole to regular IRQs. These are exactly the kind of
> exception delivery trainwrecks FRED is supposed to fix, except in this case it
> appears it doesn't :/