[PATCH v2] arm64: implement support for static call trampolines

[Quentin Perret]

Hi Ard,

On Wednesday 28 Oct 2020 at 19:41:14 (+0100), Ard Biesheuvel wrote:
> diff --git a/arch/arm64/include/asm/static_call.h b/arch/arm64/include/asm/static_call.h
> new file mode 100644
> index 000000000000..7ddf939d57f5
> --- /dev/null
> +++ b/arch/arm64/include/asm/static_call.h
> @@ -0,0 +1,32 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _ASM_STATIC_CALL_H
> +#define _ASM_STATIC_CALL_H
> +
> +/*
> + * We have to account for the possibility that the static call site may
> + * be updated to refer to a target that is out of range for an ordinary
> + * 'B' branch instruction, and so we need to pre-allocate some space for
> + * a ADRP/ADD/BR sequence.
> + */
> +#define __ARCH_DEFINE_STATIC_CALL_TRAMP(name, insn)			    \
> +	asm(".pushsection	.static_call.text, \"ax\"		\n" \
> +	    ".align		5					\n" \
> +	    ".globl		" STATIC_CALL_TRAMP_STR(name) "		\n" \
> +	    STATIC_CALL_TRAMP_STR(name) ":				\n" \
> +	    "hint 	34	/* BTI C */				\n" \
> +	    insn "							\n" \
> +	    "ret							\n" \
> +	    "nop							\n" \
> +	    "nop							\n" \
> +	    "adrp	x16, " STATIC_CALL_KEY(name) "			\n" \
> +	    "ldr	x16, [x16, :lo12:" STATIC_CALL_KEY(name) "]	\n" \
> +	    "br		x16						\n" \
> +	    ".popsection						\n")

Still trying to understand all this in details, so bear with me, but is
there any way this could be turned into the 'inline' static call
variant?

That is, could we have a piece of inline assembly at all static_call
locations that would do a branch-link, with basically x0-x18 and
x29,x30 in the clobber list (+ some have some magic to place the
parameters in the right register, like we do for SMCCC calls for
instance). That'd save a branch to the trampoline.

Ideally we'd need a way to tell the compile 'this inline assembly code
does a function call', but I'm not aware of any easy way to do that
(other that marking register clobbered + probably other things that I'm
missing).

In any case, I was looking forward to an arm64 static call port so
thanks for putting that together.

Quentin