linux-kernel - Re: [RFC PATCH 03/23] genirq: Introduce IRQF_DELIVER_AS

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Message-ID: <c000699c-7859-74ec-7944-ec484e887f60@arm.com>
Date:   Wed, 13 Jun 2018 11:06:25 +0100
From:   Marc Zyngier <marc.zyngier@....com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Julien Thierry <julien.thierry@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
        Ingo Molnar <mingo@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andi Kleen <andi.kleen@...el.com>,
        Ashok Raj <ashok.raj@...el.com>, Borislav Petkov <bp@...e.de>,
        Tony Luck <tony.luck@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>, x86@...nel.org,
        sparclinux@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, Jacob Pan <jacob.jun.pan@...el.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Levin, Alexander (Sasha Levin)" <alexander.levin@...izon.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Bartosz Golaszewski <brgl@...ev.pl>,
        Doug Berger <opendmb@...il.com>,
        Palmer Dabbelt <palmer@...ive.com>,
        iommu@...ts.linux-foundation.org
Subject: Re: [RFC PATCH 03/23] genirq: Introduce IRQF_DELIVER_AS_NMI

On 13/06/18 10:20, Thomas Gleixner wrote:
> On Wed, 13 Jun 2018, Julien Thierry wrote:
>> On 13/06/18 09:34, Peter Zijlstra wrote:
>>> On Tue, Jun 12, 2018 at 05:57:23PM -0700, Ricardo Neri wrote:
>>>> diff --git a/include/linux/interrupt.h b/include/linux/interrupt.h
>>>> index 5426627..dbc5e02 100644
>>>> --- a/include/linux/interrupt.h
>>>> +++ b/include/linux/interrupt.h
>>>> @@ -61,6 +61,8 @@
>>>>    *                interrupt handler after suspending interrupts. For
>>>> system
>>>>    *                wakeup devices users need to implement wakeup
>>>> detection in
>>>>    *                their interrupt handlers.
>>>> + * IRQF_DELIVER_AS_NMI - Configure interrupt to be delivered as
>>>> non-maskable, if
>>>> + *                supported by the chip.
>>>>    */
>>>
>>> NAK on the first 6 patches. You really _REALLY_ don't want to expose
>>> NMIs to this level.
>>>
>>
>> I've been working on something similar on arm64 side, and effectively the one
>> thing that might be common to arm64 and intel is the interface to set an
>> interrupt as NMI. So I guess it would be nice to agree on the right approach
>> for this.
>>
>> The way I did it was by introducing a new irq_state and let the irqchip driver
>> handle most of the work (if it supports that state):
>>
>> https://lkml.org/lkml/2018/5/25/181
>>
>> This has not been ACKed nor NAKed. So I am just asking whether this is a more
>> suitable approach, and if not, is there any suggestions on how to do this?
> 
> I really didn't pay attention to that as it's burried in the GIC/ARM series
> which is usually Marc's playground.

I'm working my way through it ATM now that I have some brain cycles back.

> Adding NMI delivery support at low level architecture irq chip level is
> perfectly fine, but the exposure of that needs to be restricted very
> much. Adding it to the generic interrupt control interfaces is not going to
> happen. That's doomed to begin with and a complete abuse of the interface
> as the handler can not ever be used for that.

I can only agree with that. Allowing random driver to use request_irq()
to make anything an NMI ultimately turns it into a complete mess ("hey,
NMI is *faster*, let's use that"), and a potential source of horrible
deadlocks.

What I'd find more palatable is a way for an irqchip to be able to
prioritize some interrupts based on a set of architecturally-defined
requirements, and a separate NMI requesting/handling framework that is
separate from the IRQ API, as the overall requirements are likely to
completely different.

It shouldn't have to be nearly as complex as the IRQ API, and require
much stricter requirements in terms of what you can do there (flow
handling should definitely be different).

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...