linux-kernel - Re: [PATCH] KVM/VMX: Invoke NMI non-IST entry instead of IST entry

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Date:   Tue, 4 May 2021 21:51:02 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org,
        Lai Jiangshan <laijs@...ux.alibaba.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Uros Bizjak <ubizjak@...il.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        Juergen Gross <JGross@...e.com>,
        Joerg Roedel <jroedel@...e.de>, Jian Cai <caij2003@...il.com>
Subject: Re: [PATCH] KVM/VMX: Invoke NMI non-IST entry instead of IST entry

On Tue, May 04, 2021, Paolo Bonzini wrote:
> On 04/05/21 23:23, Andy Lutomirski wrote:
> > > On May 4, 2021, at 2:21 PM, Sean Christopherson <seanjc@...gle.com> wrote:
> > > FWIW, NMIs are masked if the VM-Exit was due to an NMI.
> 
> Huh, indeed:  "An NMI causes subsequent NMIs to be blocked, but only after
> the VM exit completes".
> 
> > Then this whole change is busted, since nothing will unmask NMIs. Revert it?

SMI?  #MC?  :-)

> Looks like the easiest way out indeed.

I've no objection to reverting to intn, but what does reverting versus handling
NMI on the kernel stack have to do with NMIs being blocked on VM-Exit due to NMI?
I'm struggling mightily to connect the dots.