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 12:40:28 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Lai Jiangshan <jiangshanlai+lkml@...il.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     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>,
        Andi Kleen <ak@...ux.intel.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 26/04/21 11:33, Lai Jiangshan wrote:
> When handle_interrupt_nmi_irqoff() is called, we may lose the
> CPU-hidden-NMI-masked state due to IRET of #DB, #BP or other traps
> between VMEXIT and handle_interrupt_nmi_irqoff().
> 
> But the NMI handler in the Linux kernel*expects*  the CPU-hidden-NMI-masked
> state is still set in the CPU for no nested NMI intruding into the beginning
> of the handler.
> 
> 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.

However, I'm not sure which of the two situations is better: entering 
the NMI handler on the IST without setting the hidden NMI-blocked flag 
could be a recipe for bad things as well.

Paolo