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

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Message-ID: <20211027124744.GE54628@C02TD0UTHF1T.local>
Date:   Wed, 27 Oct 2021 13:47:44 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     unlisted-recipients:; (no To-header on input)
Cc:     Ard Biesheuvel <ardb@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        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>
Subject: Re: [PATCH 2/4] arm64: implement support for static call trampolines

On Tue, Oct 26, 2021 at 11:06:11AM +0000, David Laight wrote:
> From: Mark Rutland
> > Sent: 26 October 2021 11:37
> ...
> > My preference overall is to keep the trampoline self-contained, and I'd
> > prefer to keep the RET inline in the trampoline rather than trying to
> > factor it out so that all the control-flow is clearly in one place.
> > 
> > So I'd prefer that we have the sequence as-is:
> > 
> > | 0:	.quad 0x0
> > | 	bti	c
> > | 	< insn >
> > | 	ldr	x16, 0b
> > | 	cbz	x16, 1f
> > | 	br	x16
> > | 1:	ret
> 
> What is wrong with:
> 0:	.quad 1f
> 	bti	c
> 	< insn >
> 	ldr	x16, 0b
> 	br	x16
> 1:	bti	c
> 	ret
> 
> Self-contained and reasonably easy to read.

FWIW, that would work for me too.

Thanks,
Mark.