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

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Date:   Thu, 14 Jun 2018 19:12:13 -0700
From:   Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
To:     Marc Zyngier <marc.zyngier@....com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Julien Thierry <julien.thierry@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        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 Wed, Jun 13, 2018 at 11:06:25AM +0100, Marc Zyngier wrote:
> 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).

Marc, Julien, do you plan to actively work on this? Would you mind keeping
me in the loop? I also need this work for this watchdog. In the meantime,
I will go through Julien's patches and try to adapt it to my work.

Thanks and BR,
Ricardo