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From:   Arnd Bergmann <arnd@kernel.org>
Date:   Sat, 13 Feb 2021 00:05:43 +0100
Message-ID: <CAK8P3a0fwMe9LaXMfKjH46yvt6o-euZJZ4HXtVRPhYbKvAUPKg@mail.gmail.com>
Subject: Re: [RFC] IRQ handlers run with some high-priority interrupts(not
 NMI) enabled on some platform
To:     "Song Bao Hua (Barry Song)" <song.bao.hua@hisilicon.com>
Cc:     "tglx@linutronix.de" <tglx@linutronix.de>,
        "gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
        "arnd@arndb.de" <arnd@arndb.de>,
        "geert@linux-m68k.org" <geert@linux-m68k.org>,
        "funaho@jurai.org" <funaho@jurai.org>,
        "philb@gnu.org" <philb@gnu.org>, "corbet@lwn.net" <corbet@lwn.net>,
        "mingo@redhat.com" <mingo@redhat.com>,
        "linux-m68k@lists.linux-m68k.org" <linux-m68k@lists.linux-m68k.org>,
        "fthain@telegraphics.com.au" <fthain@telegraphics.com.au>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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On Sat, Feb 13, 2021 at 12:00 AM Song Bao Hua (Barry Song)
<song.bao.hua@hisilicon.com> wrote:
> > -----Original Message-----
> > From: Arnd Bergmann [mailto:arnd@kernel.org]
> > Sent: Saturday, February 13, 2021 11:34 AM
> > To: Song Bao Hua (Barry Song) <song.bao.hua@hisilicon.com>
> > Cc: tglx@linutronix.de; gregkh@linuxfoundation.org; arnd@arndb.de;
> > geert@linux-m68k.org; funaho@jurai.org; philb@gnu.org; corbet@lwn.net;
> > mingo@redhat.com; linux-m68k@lists.linux-m68k.org;
> > fthain@telegraphics.com.au; linux-kernel@vger.kernel.org
> > Subject: Re: [RFC] IRQ handlers run with some high-priority interrupts(not NMI)
> > enabled on some platform
> >
> > On Fri, Feb 12, 2021 at 2:18 AM Song Bao Hua (Barry Song)
> > <song.bao.hua@hisilicon.com> wrote:
> >
> > > So I am requesting comments on:
> > > 1. are we expecting all interrupts except NMI to be disabled in irq handler,
> > > or do we actually allow some high-priority interrupts between low and NMI
> > to
> > > come in some platforms?
> >
> > I tried to come to an answer but this does not seem particularly well-defined.
> > There are a few things I noticed:
> >
> > - going through the local_irq_save()/restore() implementations on all
> >   architectures, I did not find any other ones besides m68k that leave
> >   high-priority interrupts enabled. I did see that at least alpha and openrisc
> >   are designed to support that in hardware, but the code just leaves the
> >   interrupts disabled.
>
> The case is a little different. Explicit local_irq_save() does disable all
> high priority interrupts on m68k. The only difference is arch_irqs_disabled()
> of m68k will return true while low-priority interrupts are masked and high
> -priority are still open. M68k's hardIRQ also runs in this context with high
> priority interrupts enabled.

My point was that on most other architectures, local_irq_save()/restore()
always disables/enables all interrupts, while on m68k it restores the
specific level they were on before. On alpha, it does the same as on m68k,
but then the top-level interrupt handler just disables them all before calling
into any other code.

It's possible that I missed some other implementation doing the same
as m68k, as this code is fairly subtle on some architectures.

        Arnd