linux-kernel - Re: [PATCH 2/4] x86/entry: Use asm_noist_exc

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Message-ID: <87wnseis8v.ffs@nanos.tec.linutronix.de>
Date:   Tue, 04 May 2021 14:43:44 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org
Cc:     Lai Jiangshan <laijs@...ux.alibaba.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.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>,
        Maxim Levitsky <mlevitsk@...hat.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 2/4] x86/entry: Use asm_noist_exc_nmi() for NMI in early booting stage

On Mon, May 03 2021 at 23:45, Thomas Gleixner wrote:
> The problem was introduced in 4.14 with b70543a0b2b6 ("x86/idt: Move
> regular trap init to tables").
>
> Before that trap_init() installed an IST gate right away, but looking
> deeper this was broken forever because there is a hen and egg problem.
>
> ISTs only work after TSS is initialized and the ordering here is:
>
> trap_init()
>   init_idt()
>   cpu_init()
>     init_tss()
>
> So the original code had a race window between init_idt() and
> init_tss(). Any IST using exception in that window goes south because
> TSS is not initialized.
>
> b70543a0b2b6 traded the above with that NMI issue. All other
> exceptions are fine...
>
> I'll think about it tomorrow some more...

It does not really matter which way around we do it. Even if we do that
noist dance then still any NMI hitting _before_ init_idt() is going to
lala land. So having this tiny step in between is more or less cosmetic.

And just for completness sake, I don't see a reason why we have to set
up the idt gates _before_ the TSS muck, i.e. before cpu_init().

The only thing cpu_init() needs working which is not installed in the
early_idt is #GP because some cpu init code uses rd/wrmsrl_safe(). But
that's pretty much all of it.

So this wants a proper cleanup and not some paper over it with an extra
step and I don't see a reason why any of this should be backported
simply because it does not matter at all whether the early idt which
only populates a few essential gates is active for a bit longer.

So what we need is a solution for that KVM wreckage but that can be
stand alone.

Thanks,

        tglx