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

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Date:   Wed, 05 May 2021 02:00:10 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        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 at 23:56, Paolo Bonzini wrote:
> 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.

Well, looks good is one thing.

It would be more helpful if someone would actually review and/or test it.

Thanks,

        tglx