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

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Message-ID: <1342024090.14828.45.camel@gandalf.stny.rr.com>
Date:	Wed, 11 Jul 2012 12:28:10 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
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

On Wed, 2012-07-11 at 12:22 -0400, Steven Rostedt wrote:
> On Tue, 2012-07-03 at 17:29 +0900, Masami Hiramatsu wrote:
> 
> 
> > 
> > > +	/* 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.
> 
> Ah, because the addq will modify flags :-/
> 
> Grumble, I guess I should implement this, even though it will make it a
> little more complex. I thought it was better to restore flags
> explicitly, but that's not the case.
> 

I know why I did this. Do you want kprobes to be able to modify flags?
If so, then I need to add, before the restore:

	movq EFLAGS(%rsp), %rax
	movq %rax, SS(%rsp)

-- Steve


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