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From: Steven Rostedt <rostedt@goodmis.org>
To: Lai Jiangshan <jiangshanlai+lkml@gmail.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	kvm@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Uros Bizjak <ubizjak@gmail.com>, Andi Kleen <ak@linux.intel.com>,
	Andy Lutomirski <luto@kernel.org>,
	Sean Christopherson <seanjc@google.com>,
	Maxim Levitsky <mlevitsk@redhat.com>
Subject: Re: [PATCH v2 2/2] KVM: VMX: Invoke NMI handler via indirect call instead of INTn
Date: Mon, 26 Apr 2021 21:00:20 -0400	[thread overview]
Message-ID: <20210426210020.417e3cfc@oasis.local.home> (raw)
In-Reply-To: <CAJhGHyDrAwKO1iht=d0j+OKD1U7e1fzLminudxo2sPHbF53TKQ@mail.gmail.com>

On Tue, 27 Apr 2021 08:54:37 +0800
Lai Jiangshan <jiangshanlai+lkml@gmail.com> wrote:

> > 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.  
> 
> The change makes the ASM NMI entry called on the kernel stack.  But the
> ASM NMI entry expects it on the IST stack and it plays with "NMI executing"
> variable on the IST stack.  In this change, the stranded ASM NMI entry
> will use the wrong/garbage "NMI executing" variable on the kernel stack
> and may do some very wrong thing.

I missed this detail.

> 
> Sorry, in my reply, "the NMI handler" meant to be the ASM entry installed
> on the IDT table which really expects to be NMI-masked at the beginning.
> 
> The C NMI handler can handle the case of nested NMIs, which is useful
> here.  I think we should change it to call the C NMI handler directly
> here as Andy Lutomirski suggested:

Yes, because that's the way x86_32 works.

> 
> On Mon, Apr 26, 2021 at 11:09 PM Andy Lutomirski <luto@amacapital.net> wrote:
> > The C NMI code has its own reentrancy protection and has for years.
> > It should work fine for this use case.  
> 
> I think this is the right way.

Agreed.

-- Steve

  reply	other threads:[~2021-04-27  1:01 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-15 19:15 [PATCH v2 0/2] KVM: VMX: Clean up IRQ/NMI handling Sean Christopherson
2020-09-15 19:15 ` [PATCH v2 1/2] KVM: VMX: Move IRQ invocation to assembly subroutine Sean Christopherson
2020-09-15 19:27   ` Josh Poimboeuf
2020-09-15 19:38   ` Uros Bizjak
2020-09-15 19:15 ` [PATCH v2 2/2] KVM: VMX: Invoke NMI handler via indirect call instead of INTn Sean Christopherson
2021-04-26  9:33   ` Lai Jiangshan
2021-04-26 10:40     ` Paolo Bonzini
2021-04-26 11:44       ` Maxim Levitsky
2021-04-26 13:59         ` Steven Rostedt
2021-04-26 14:51       ` Andi Kleen
2021-04-26 15:09         ` Andy Lutomirski
2021-04-27  0:54       ` Lai Jiangshan
2021-04-27  1:00         ` Steven Rostedt [this message]
2021-04-27  7:05         ` Paolo Bonzini
2021-04-30  2:56           ` Lai Jiangshan
2020-09-22 13:38 ` [PATCH v2 0/2] KVM: VMX: Clean up IRQ/NMI handling Paolo Bonzini

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