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

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Message-Id: <38B9D60F-F24F-4910-B2DF-2A57F1060452@amacapital.net>
Date:   Tue, 4 May 2021 14:23:37 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        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 May 4, 2021, at 2:21 PM, Sean Christopherson <seanjc@...gle.com> wrote:
> 
> On Tue, May 04, 2021, Paolo Bonzini wrote:
>>> On 04/05/21 23:05, Maxim Levitsky wrote:
>>> Does this mean that we still rely on hardware NMI masking to be activated?
>> 
>> No, the NMI code already handles reentrancy at both the assembly and C
>> levels.
>> 
>>> Or in other words, that is we still can't have an IRET between VM exit and
>>> the entry to the NMI handler?
>> 
>> No, because NMIs are not masked on VM exit.  This in fact makes things
>> potentially messy; unlike with AMD's CLGI/STGI, only MSRs and other things
>> that Intel thought can be restored atomically with the VM exit.
> 
> FWIW, NMIs are masked if the VM-Exit was due to an NMI.

Then this whole change is busted, since nothing will unmask NMIs. Revert it?