linux-kernel - Re: [PATCH 5/6] ftrace/x86: Add separate function to save regs

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Date:	Tue, 03 Jul 2012 17:29:13 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"H. Peter Anvin" <hpa@...or.com>, yrl.pp-manager.tt@...achi.com
Subject: Re: [PATCH 5/6] ftrace/x86: Add separate function to save regs

(2012/07/03 5:03), Steven Rostedt wrote:
> From: Steven Rostedt <srostedt@...hat.com>
> 
> Add a way to have different functions calling different trampolines.
> If a ftrace_ops wants regs saved on the return, then have only the
> functions with ops registered to save regs. Functions registered by
> other ops would not be affected, unless the functions overlap.
> 
> If one ftrace_ops registered functions A, B and C and another ops
> registered fucntions to save regs on A, and D, then only functions
> A and D would be saving regs. Function B and C would work as normal.
> Although A is registered by both ops: normal and saves regs; this is fine
> as saving the regs is needed to satisfy one of the ops that calls it
> but the regs are ignored by the other ops function.
> 
> x86_64 implements the full regs saving, and i386 just passes a NULL
> for regs to satisfy the ftrace_ops passing. Where an arch must supply
> both regs and ftrace_ops parameters, even if regs is just NULL.
> 
> It is OK for an arch to pass NULL regs. All function trace users that
> require regs passing must add the flag FTRACE_OPS_FL_SAVE_REGS when
> registering the ftrace_ops. If the arch does not support saving regs
> then the ftrace_ops will fail to register. The flag
> FTRACE_OPS_FL_SAVE_REGS_IF_SUPPORTED may be set that will prevent the
> ftrace_ops from failing to register. In this case, the handler may
> either check if regs is not NULL or check if ARCH_SUPPORTS_FTRACE_SAVE_REGS.
> If the arch supports passing regs it will set this macro and pass regs
> for ops that request them. All other archs will just pass NULL.

This is nice for me :)
It helps me to maintain kprobes on ftrace.

[...]
> @@ -98,6 +111,67 @@ GLOBAL(ftrace_stub)
>  	retq
>  END(ftrace_caller)
>  
> +ENTRY(ftrace_regs_caller)
> +	/* Save the current flags before compare (in SS location)*/
> +	pushfq
> +
> +	/* Check if tracing was disabled (quick check) */
> +	cmpl $0, function_trace_stop
> +	jne  ftrace_restore_flags
> +
> +	/* skip=8 to skip flags saved in SS */
> +	ftrace_caller_setup 8
> +
> +	/* Save the rest of pt_regs */
> +	movq %r15, R15(%rsp)
> +	movq %r14, R14(%rsp)
> +	movq %r13, R13(%rsp)
> +	movq %r12, R12(%rsp)
> +	movq %r11, R11(%rsp)
> +	movq %r10, R10(%rsp)
> +	movq %rbp, RBP(%rsp)
> +	movq %rbx, RBX(%rsp)
> +	/* Copy saved flags */
> +	movq SS(%rsp), %rcx
> +	movq %rcx, EFLAGS(%rsp)
> +	/* Kernel segments */
> +	movq $__KERNEL_DS, %rcx
> +	movq %rcx, SS(%rsp)
> +	movq $__KERNEL_CS, %rcx
> +	movq %rcx, CS(%rsp)
> +	/* Stack - skipping return address */
> +	leaq SS+16(%rsp), %rcx
> +	movq %rcx, RSP(%rsp)
> +
> +	/* regs go into 4th parameter */
> +	leaq (%rsp), %rcx
> +
> +GLOBAL(ftrace_regs_call)
> +	call ftrace_stub
> +
> +	/* restore the rest of pt_regs */
> +	movq R15(%rsp), %r15
> +	movq R14(%rsp), %r14
> +	movq R13(%rsp), %r13
> +	movq R12(%rsp), %r12
> +	movq R10(%rsp), %r10
> +	movq RBP(%rsp), %rbp
> +	movq RBX(%rsp), %rbx
> +


> +	/* Restore flags */
> +	pushq EFLAGS(%rsp)
> +	popfq
> +
> +	MCOUNT_RESTORE_FRAME

Here, if MCOUNT_RESTORE_FRAME has skip too, I think you don't
need to restore flags before restoring other registers, like
below;

	MCOUNT_RESTORE_FRAME 8
	popfq

And also, this will prevent to modify flags before return by
addq in MCOUNT_RESTORE_FRAME.

> +
> +	jmp ftrace_return
> +ftrace_restore_flags:
> +	popfq
> +	jmp  ftrace_stub
> +
> +END(ftrace_regs_caller)
> +

Thank you,


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


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