Messages in this thread | | | From | Ard Biesheuvel <> | Date | Mon, 25 Oct 2021 16:31:34 +0200 | Subject | Re: [PATCH 2/4] arm64: implement support for static call trampolines |
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On Mon, 25 Oct 2021 at 16:25, David Laight <David.Laight@aculab.com> 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.
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