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

[Andrew Cooper]

On 11/11/2022 14:23, Peter Zijlstra wrote:
> On Fri, Nov 11, 2022 at 01:48:26PM +0100, Paolo Bonzini wrote:
>> On 11/11/22 13:19, Peter Zijlstra wrote:
>>> On Fri, Nov 11, 2022 at 01:04:27PM +0100, Paolo Bonzini wrote:
>>>> 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.

On server SKUs.  Client only got "virtual interrupt processing" fairly
recently IIRC, which is the CPU-side property which matters.

> (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...

The only FRED-compatible fix is to send a self-NMI, because because you
may need a CSL change too.

VT-x *does* hold the NMI latch (for VMEXIT_REASON NMI), so it's self-NMI
and then enable_nmi()s.

Except the IRET to self won't work - it will need to be ERETS-to-self. 
Which I think is fine.

But what isn't fine is the fact that a self-NMI doesn't deliver
synchronously, so you need to wait until it is pending, before enabling
NMIs.  (Well, actually you need to ensure that it's definitely delivered
before re-entering the VM).

And I'm totally out of ideas here...

~Andrew