RE: [PATCH v5 34/34] KVM: x86/vmx: execute "int $2" to handle NMI in NMI caused VM exits when FRED is enabled

[Li, Xin3]

> Organization aside, this seems to defeat the purpose of _not_ unconditionally
> unmasking NMIs on ERET since the kernel assumes any random "int $2" is coming
> from KVM after an NMI VM-Exit.

I'm a bit confused.  KVM VMX is the only component needing to execute "int $2"
and it surely has NMI blocked after an NMI VM-exit.

> Eww, and "int $2" doesn't even go directly to fred_exc_nmi(), it trampolines
> through fred_sw_interrupt_kernel() first.  Looks like "int $2" from userspace gets
> routed to a #GP, so at least that bit is handled.

FRED does a 2-level dispatch, unless an event handler is on a hot path,
we don't promote its handling.  NMI seems not a frequent event.

> I'm not dead set against the proposed approach, but IMO it's not obviously better
> than a bit of assembly to have a more direct call into the NMI handler.

I will give it a shot.