linux-kernel - Re: [PATCH v2 2/2] KVM: VMX: Invoke NMI handler via indirect call instead of INTn

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Date:   Mon, 26 Apr 2021 08:09:46 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Lai Jiangshan <jiangshanlai+lkml@...il.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Uros Bizjak <ubizjak@...il.com>,
        Andy Lutomirski <luto@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH v2 2/2] KVM: VMX: Invoke NMI handler via indirect call instead of INTn


> On Apr 26, 2021, at 7:51 AM, Andi Kleen <ak@...ux.intel.com> wrote:
> 
> 
>> 
>>> The original code "int $2" can provide the needed CPU-hidden-NMI-masked
>>> when entering #NMI, but I doubt it about this change.
>> 
>> How would "int $2" block NMIs?  The hidden effect of this change (and I
>> should have reviewed better the effect on the NMI entry code) is that the
>> call will not use the IST anymore.
> 
> My understanding is that int $2 does not block NMIs.
> 
> So reentries might have been possible.
> 

The C NMI code has its own reentrancy protection and has for years.  It should work fine for this use case.

> -Andi