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

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Date:   Wed, 11 May 2022 12:23:57 +0000
From:   "Hawkins, Nick" <nick.hawkins@....com>
To:     Will Deacon <will@...nel.org>, Arnd Bergmann <arnd@...db.de>
CC:     "Verdun, Jean-Marie" <verdun@....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.

Hello Arnd,

Given Will's input would you like me to still use the ex_table method?

Thanks,

-Nick Hawkins