linux-kernel - RE: [PATCH 2/4] arm64: implement support for static call trampolines

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Date:   Mon, 25 Oct 2021 14:25:53 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Frederic Weisbecker' <frederic@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ard Biesheuvel <ardb@...nel.org>
CC:     LKML <linux-kernel@...r.kernel.org>,
        James Morse <james.morse@....com>,
        Quentin Perret <qperret@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>
Subject: RE: [PATCH 2/4] arm64: implement support for static call trampolines

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).

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

Or am I missing something?

	David

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