Re: [PATCH 2/4] arm64: implement support for static call trampolines

[Ard Biesheuvel]

On Mon, 25 Oct 2021 at 16:25, David Laight <David.Laight@xxxxxxxxxx> wrote:
>
> From: Frederic Weisbecker
> > Sent: 25 October 2021 13:21
> >
> > Implement arm64 support for the 'unoptimized' static call variety, which
> > routes all calls through a single trampoline that is patched to perform a
> > tail call to the selected function.
> >
> > It is expected that the direct branch instruction will be able to cover
> > the common case. However, given that static call targets may be located
> > in modules loaded out of direct branching range, we need a fallback path
> > that loads the address into R16 and uses a branch-to-register (BR)
> > instruction to perform an indirect call.
> >
> ...
> > +void arch_static_call_transform(void *site, void *tramp, void *func, bool tail)
> > +{
> > +     /*
> > +      * -0x8 <literal>
> > +      *  0x0 bti c           <--- trampoline entry point
> > +      *  0x4 <branch or nop>
> > +      *  0x8 ldr x16, <literal>
> > +      *  0xc cbz x16, 20
> > +      * 0x10 br x16
> > +      * 0x14 ret
> > +      */
>
> Since the 'ldr x16, <literal>' is just a 32bit constant
> (for a pc-relative load).
>

I don't follow. Are you saying it is a 32-bit opcode? This applies to
all AArch64 opcodes.

> Can't you save a word by making offset 0x4 <branch or ldr x16, <literal>> ?
>
> Or am I missing something?
>

On arm64, we can only patch NOPs into branch instructions or vice
versa, or we'd have to run the whole thing under stop_machine() to
ensure that other cores don't fetch garbage.