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Date:   Tue, 21 Sep 2021 09:09:10 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Frederic Weisbecker <frederic@kernel.org>
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Ard Biesheuvel <ardb@kernel.org>,
        James Morse <james.morse@arm.com>,
        Quentin Perret <qperret@google.com>,
        Mark Rutland <mark.rutland@arm.com>,
        christophe.leroy@csgroup.eu
Subject: Re: [PATCH 2/4] arm64: implement support for static call trampolines
Message-ID: <YUmFFvZCb2yXn3os@hirez.programming.kicks-ass.net>
References: <20210920233237.90463-1-frederic@kernel.org>
 <20210920233237.90463-3-frederic@kernel.org>
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On Tue, Sep 21, 2021 at 01:32:35AM +0200, Frederic Weisbecker wrote:

> +#define __ARCH_DEFINE_STATIC_CALL_TRAMP(name, target)			    \
> +	asm("	.pushsection	.static_call.text, \"ax\"		\n" \
> +	    "	.align		3					\n" \
> +	    "	.globl		" STATIC_CALL_TRAMP_STR(name) "		\n" \
> +	    STATIC_CALL_TRAMP_STR(name) ":				\n" \
> +	    "	hint 	34	/* BTI C */				\n" \
> +	    "	adrp	x16, 1f						\n" \
> +	    "	ldr	x16, [x16, :lo12:1f]				\n" \
> +	    "	cbz	x16, 0f						\n" \
> +	    "	br	x16						\n" \
> +	    "0:	ret							\n" \
> +	    "	.popsection						\n" \
> +	    "	.pushsection	.rodata, \"a\"				\n" \
> +	    "	.align		3					\n" \
> +	    "1:	.quad		" target "				\n" \
> +	    "	.popsection						\n")

So I like what Christophe did for PPC32:

  https://lkml.kernel.org/r/6ec2a7865ed6a5ec54ab46d026785bafe1d837ea.1630484892.git.christophe.leroy@csgroup.eu

Where he starts with an unconditional jmp and uses that IFF the offset
fits and only does the data load when it doesn't. Ard, woulnd't that
also make sense on ARM64? I'm thinking most in-kernel function pointers
would actually fit, it's just the module muck that gets to have too
large pointers, no?

> +#define ARCH_DEFINE_STATIC_CALL_TRAMP(name, func)			\
> +	__ARCH_DEFINE_STATIC_CALL_TRAMP(name, #func)
> +
> +#define ARCH_DEFINE_STATIC_CALL_NULL_TRAMP(name)			\
> +	__ARCH_DEFINE_STATIC_CALL_TRAMP(name, "0x0")