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

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Message-ID: <PH0PR84MB171846CCF58FC36A39C39EF388C99@PH0PR84MB1718.NAMPRD84.PROD.OUTLOOK.COM>
Date:   Tue, 10 May 2022 13:41:10 +0000
From:   "Hawkins, Nick" <nick.hawkins@....com>
To:     Arnd Bergmann <arnd@...db.de>,
        "Verdun, Jean-Marie" <verdun@....com>
CC:     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.

I will pursue the method you have recommended Arnd.

Thank you for the feedback as always.

-Nick Hawkins