linux-kernel - Re: [for-next][PATCH 1/4] ftrace/x86: Add dynamic allocated trampoline for ftrace

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Date:	Fri, 31 Oct 2014 14:19:03 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jiri Kosina <jkosina@...e.cz>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Vojtech Pavlik <vojtech@...e.cz>,
	Seth Jennings <sjenning@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [for-next][PATCH 1/4] ftrace/x86: Add dynamic allocated trampoline
 for ftrace_ops

Hi Steven,

(2014/10/28 3:27), Steven Rostedt wrote:
> +static unsigned long create_trampoline(struct ftrace_ops *ops)
> +{
> +	unsigned const char *jmp;
> +	unsigned long start_offset;
> +	unsigned long end_offset;
> +	unsigned long op_offset;
> +	unsigned long offset;
> +	unsigned long size;
> +	unsigned long ip;
> +	unsigned long *ptr;
> +	void *trampoline;
> +	/* 48 8b 15 <offset> is movq <offset>(%rip), %rdx */
> +	unsigned const char op_ref[] = { 0x48, 0x8b, 0x15 };
> +	union ftrace_op_code_union op_ptr;
> +	int ret;
> +
> +	if (ops->flags & FTRACE_OPS_FL_SAVE_REGS) {
> +		start_offset = (unsigned long)ftrace_regs_caller;
> +		end_offset = (unsigned long)ftrace_regs_caller_end;
> +		op_offset = (unsigned long)ftrace_regs_caller_op_ptr;
> +	} else {
> +		start_offset = (unsigned long)ftrace_caller;
> +		end_offset = (unsigned long)ftrace_caller_end;
> +		op_offset = (unsigned long)ftrace_caller_op_ptr;
> +	}
> +
> +	size = end_offset - start_offset;
> +
> +	/*
> +	 * Allocate enough size to store the ftrace_caller code,
> +	 * the jmp to ftrace_return, as well as the address of
> +	 * the ftrace_ops this trampoline is used for.
> +	 */
> +	trampoline = alloc_tramp(size + MCOUNT_INSN_SIZE + sizeof(void *));
> +	if (!trampoline)
> +		return 0;
> +
> +	/* Copy ftrace_caller onto the trampoline memory */
> +	ret = probe_kernel_read(trampoline, (void *)start_offset, size);
> +	if (WARN_ON(ret < 0)) {
> +		tramp_free(trampoline);
> +		return 0;
> +	}
> +
> +	ip = (unsigned long)trampoline + size;
> +
> +	/* The trampoline ends with a jmp to ftrace_return */
> +	jmp = ftrace_jmp_replace(ip, (unsigned long)ftrace_return);
> +	memcpy(trampoline + size, jmp, MCOUNT_INSN_SIZE);
> +
> +	/*
> +	 * The address of the ftrace_ops that is used for this trampoline
> +	 * is stored at the end of the trampoline. This will be used to
> +	 * load the third parameter for the callback. Basically, that
> +	 * location at the end of the trampoline takes the place of
> +	 * the global function_trace_op variable.
> +	 */
> +
> +	ptr = (unsigned long *)(trampoline + size + MCOUNT_INSN_SIZE);
> +	*ptr = (unsigned long)ops;
> +
> +	op_offset -= start_offset;
> +	memcpy(&op_ptr, trampoline + op_offset, OP_REF_SIZE);
> +
> +	/* Are we pointing to the reference? */
> +	if (WARN_ON(memcmp(op_ptr.op, op_ref, 3) != 0)) {
> +		tramp_free(trampoline);
> +		return 0;
> +	}
> +
> +	/* Load the contents of ptr into the callback parameter */
> +	offset = (unsigned long)ptr;
> +	offset -= (unsigned long)trampoline + op_offset + OP_REF_SIZE;
> +
> +	op_ptr.offset = offset;
> +
> +	/* put in the new offset to the ftrace_ops */
> +	memcpy(trampoline + op_offset, &op_ptr, OP_REF_SIZE);

Would we better call flush_icache_range here?

Thank you,

> +
> +	/* ALLOC_TRAMP flags lets us know we created it */
> +	ops->flags |= FTRACE_OPS_FL_ALLOC_TRAMP;
> +
> +	return (unsigned long)trampoline;
> +}


-- 
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Research Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@...achi.com


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