linux-kernel - Re: [PATCH V2] x86/entry/64: De-Xen-ify our NMI code further

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Date:   Mon, 25 Jan 2021 13:16:24 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Lai Jiangshan <laijs@...ux.alibaba.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH V2] x86/entry/64: De-Xen-ify our NMI code further

On Mon, 25 Jan 2021 09:51:45 -0800
Andy Lutomirski <luto@...capital.net> wrote:

> > The problem I see with this is that exc_nmi is called with the thread
> > stack, if it were to take an exception, NMIs would be enabled allowing for
> > a nested NMI to run. From what I can tell, I don't see anything stopping
> > that NMI from executing over the currently running NMI. That is, this means
> > that NMI handlers are now re-entrant.  
> 
> That was intentional in my part. The C code checks for this condition and handles it, just like it does on 32-bit kernels.

I vaguely remember you implementing this, and me reviewing it. I'm getting
to that age that there's less and less I remember doing :-/

I'll include a comment about that in my rewrite. But first, I'll re-review
your changes to make sure there's no one offs that happen with the mixture
of nmi stack handling and the x86_32 version doing the same thing.

Thanks for the reminder.

-- Steve