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

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Message-ID: <5d7ca301-a0b2-d389-3bc2-feb304c9f5b5@redhat.com>
Date:   Tue, 4 May 2021 23:56:29 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.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 04/05/21 23:51, Sean Christopherson wrote:
> 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?
>> 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.

Nah, you're right: vmx_do_interrupt_nmi_irqoff will not call the handler 
directly, rather it calls the IDT entrypoint which *will* do an IRET and 
unmask NMIs.  I trusted Andy too much on this one. :)

Thomas's posted patch ("[PATCH] KVM/VMX: Invoke NMI non-IST entry 
instead of IST entry") looks good.

Paolo