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

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Date:   Tue, 10 May 2022 15:01:26 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     "Verdun, Jean-Marie" <verdun@....com>
Cc:     Arnd Bergmann <arnd@...db.de>,
        "Hawkins, Nick" <nick.hawkins@....com>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        Will Deacon <will@...nel.org>,
        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 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.

       Arnd