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From:   "Li, Xin3" <xin3.li@intel.com>
To:     "Christopherson,, Sean" <seanjc@google.com>,
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        "pbonzini@redhat.com" <pbonzini@redhat.com>,
        "Shankar, Ravi V" <ravi.v.shankar@intel.com>
Subject: RE: [PATCH v5 34/34] KVM: x86/vmx: execute "int $2" to handle NMI in
 NMI caused VM exits when FRED is enabled
Thread-Topic: [PATCH v5 34/34] KVM: x86/vmx: execute "int $2" to handle NMI in
 NMI caused VM exits when FRED is enabled
Thread-Index: AQHZUKI3WfAyEb/jCU66DZKXu8CGnq8HLBKAgABX0ICAAArqgIAACW+w
Date:   Thu, 23 Mar 2023 00:26:25 +0000
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> Organization aside, this seems to defeat the purpose of _not_ uncondition=
ally
> unmasking NMIs on ERET since the kernel assumes any random "int $2" is co=
ming
> 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 trampoli=
nes
> through fred_sw_interrupt_kernel() first.  Looks like "int $2" from users=
pace 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 obviousl=
y better
> than a bit of assembly to have a more direct call into the NMI handler.

I will give it a shot.