Re: [PATCH -v5mkII 13/17] arm/ftrace: Use __patch_text()

Arnd Bergmann
2020-01-08T14:22:55Z

On Wed, Jan 8, 2020 at 3:16 PM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> On Wed, 8 Jan 2020 13:22:52 +0100
> Arnd Bergmann <arnd@arndb.de> wrote:
>
> > The problem is that we don't have a BE32 definition of
> > __opcode_to_mem_thumb32, mostly because no hardware
> > supports that.
> >
> > One possible workaround is a big ugly:
> >
> > diff --git a/arch/arm/kernel/patch.c b/arch/arm/kernel/patch.c
> > index d0a05a3bdb96..1067fd122897 100644
> > --- a/arch/arm/kernel/patch.c
> > +++ b/arch/arm/kernel/patch.c
> > @@ -90,9 +90,11 @@ void __kprobes __patch_text_real(void *addr,
> > unsigned int insn, bool remap)
> >
> > size = sizeof(u32);
> > } else {
> > +#ifdef CONFIG_THUMB2_KERNEL
> > if (thumb2)
>
> If we change the above to:
>
> if (IS_ENABLED(CONFIG_THUMB2_KERNEL) && thumb2)
>
> Would the compiler optmize out __opcode_to_mem_thumb32(). We would need
> to add a declaration for it, but wont need to define it. At least we
> wont have nasty #ifdef logic in the code.

If we add a declaration for it, I think no change to patch.c is needed,
as 'thumb2' already has compile-time constant value equal to
IS_ENABLED(CONFIG_THUMB2_KERNEL).

I'll try it out.

Arnd