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Date:   Tue, 15 Nov 2022 11:27:03 +0000
From:   Will Deacon <will@kernel.org>
To:     Mark Rutland <mark.rutland@arm.com>
Cc:     linux-kernel@vger.kernel.org, catalin.marinas@arm.com,
        linux-arm-kernel@lists.infradead.org, mhiramat@kernel.org,
        revest@chromium.org, rostedt@goodmis.org
Subject: Re: [PATCH v2 4/4] ftrace: arm64: move from REGS to ARGS
Message-ID: <20221115112701.GB32523@willie-the-truck>
References: <20221103170520.931305-1-mark.rutland@arm.com>
 <20221103170520.931305-5-mark.rutland@arm.com>
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On Thu, Nov 03, 2022 at 05:05:20PM +0000, Mark Rutland wrote:
> This commit replaces arm64's support for FTRACE_WITH_REGS with support
> for FTRACE_WITH_ARGS. This removes some overhead and complexity, and
> removes some latent issues with inconsistent presentation of struct
> pt_regs (which can only be reliably saved/restored at exception
> boundaries).

[...]

> @@ -78,10 +77,71 @@ static inline unsigned long ftrace_call_adjust(unsigned long addr)
>  	return addr;
>  }
>  
> -#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
> +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_ARGS
>  struct dyn_ftrace;
>  struct ftrace_ops;
> -struct ftrace_regs;
> +
> +#define arch_ftrace_get_regs(regs) NULL
> +
> +struct ftrace_regs {
> +	/* x0 - x8 */
> +	unsigned long regs[9];
> +	unsigned long __unused;
> +
> +	unsigned long fp;
> +	unsigned long lr;
> +
> +	unsigned long sp;
> +	unsigned long pc;
> +};
> +
> +static __always_inline unsigned long
> +ftrace_regs_get_instruction_pointer(const struct ftrace_regs *fregs)
> +{
> +	return fregs->pc;
> +}
> +
> +static __always_inline void
> +ftrace_regs_set_instruction_pointer(struct ftrace_regs *fregs,
> +				    unsigned long pc)
> +{
> +	fregs->pc = pc;
> +}
> +
> +static __always_inline unsigned long
> +ftrace_regs_get_stack_pointer(const struct ftrace_regs *fregs)
> +{
> +	return fregs->sp;
> +}
> +
> +static __always_inline unsigned long
> +ftrace_regs_get_argument(struct ftrace_regs *fregs, unsigned int n)
> +{
> +	if (n < 8)
> +		return fregs->regs[n];

Where does this '8' come from?

Will