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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Thu, 16 Mar 2017 11:19:44 -0700
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Subject: Re: [PATCH 4/5 v3] ftrace/x86_32: Clean up ftrace_regs_caller
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Josh Poimboeuf <jpoimboe@redhat.com>
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On Thu, Mar 16, 2017 at 11:09 AM, Steven Rostedt <rostedt@goodmis.org> wrote:
> +
> +       /* Since we don't care about cs, move flags there to simplify return */
> +       movl    14*4(%esp), %eax
> +       movl    %eax, 13*4(%esp)
> +
> +       /* Move return ip back to its original location */
> +       movl    12*4(%esp), %eax
> +       movl    %eax, 14*4(%esp)

Could this perhaps be removed entirely?

The return code could instead do:

        ... restore all the normal registers ..

        # Now restore flags that is under the return address and our
fake __KERNEL_CS
        pushl 8(%esp)
        popfl

        # and then return, skipping __KERNEL_CS and %flasg
        ret $8

which is smaller and simpler than (again) playing games with stack entries.

                Linus