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

[Will Deacon]

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@xxxxxxx> 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