linux-kernel - Re: [PATCH v1] ARM: A9: Add ARM ERRATA 764319 workaround

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Date:   Tue, 10 May 2022 15:11:24 +0100
From:   Will Deacon <will@...nel.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     "Verdun, Jean-Marie" <verdun@....com>,
        "Hawkins, Nick" <nick.hawkins@....com>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        Mark Rutland <mark.rutland@....com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Joel Stanley <joel@....id.au>
Subject: Re: [PATCH v1] ARM: A9: Add ARM ERRATA 764319 workaround

On Tue, May 10, 2022 at 03:01:26PM +0200, Arnd Bergmann wrote:
> On Tue, May 10, 2022 at 1:53 PM Verdun, Jean-Marie <verdun@....com> wrote:
> >
> > Hi Arnd,
> >
> > >    Hi Nick,
> >
> > >    This seems a bit more complex than necessary. Can't you just use a custom
> > >    inline asm with an ex_table entry to catch the fault? Have a look at
> > >    __get_user_asm() for an example.
> > >
> > >           Arnd
> >
> > We got inspired from debug_reg_hook within the same source file
> >( ./arch/arm/kernel/hw_breakpoint.c ). We chose that path to keep coherency
> > within the source code. We can implement the same fix by using an ex_table
> > entry, but this will create two different ways at catching unknown instruction
> > within the same source file. Will that be ok ?
> 
> I got a little lost trying to find where the breakpoint instruction comes
> from that gets trapped here, but I would guess that they had to do this
> using an undef_hook because the ex_table approach does not work
> there for some reason.
> 
> I would still pick the ex_table method here if that works.

IIRC, the ex_table handlers are called only for data aborts and are intended
to be used to handle cases where we take a fault on a memory access (e.g.
translation fault). In this case, we're taking an undefined instruction
exception on a cp14 access and so the undef_hook is the right thing to use.

Will