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

[Li, Xin3]

> > > > > > On Intel you can optionally make it hold onto IRQs, but NMIs
> > > > > > are always eaten by the VMEXIT and have to be reinjected manually.
> > > > >
> > > > > That 'optionally' thing worries me -- as in, KVM is currently
> > > > > opting-out?
> > > >
> > > > Yes, because "If the “process posted interrupts” VM-execution
> > > > control is 1, the “acknowledge interrupt on exit” VM-exit control
> > > > is 1" (SDM 26.2.1.1, checks on VM-Execution Control Fields).  Ipse
> > > > dixit.  Posted interrupts are available and used on all processors since I
> think Ivy Bridge.
> > >
> > > (imagine the non-coc compliant reaction here)
> > >
> > > 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.
> 
> 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.

What you said here implies that we have this problem in the existing code.
Because a fake iret stack is created to call the NMI handler in the IDT NMI
descriptor, which lastly executes the IRET instruction.

> 
> 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 :/