Re: [RFC][PATCH] ftrace/x86: Remove mcount support

[Peter Zijlstra]

On Thu, May 09, 2019 at 03:49:02PM -0400, Steven Rostedt wrote:
> 
> From: "Steven Rostedt (VMware)" <rostedt@xxxxxxxxxxx>
> 
> There's two methods of enabling function tracing in Linux on x86. One is
> with just "gcc -pg" and the other is "gcc -pg -mfentry". The former will use
> calls to a special function "mcount" after the frame is set up in all C
> functions. The latter will add calls to a special function called "fentry"
> as the very first instruction of all C functions.
> 
> At compile time, there is a check to see if gcc supports, -mfentry, and if
> it does, it will use that, because it is more versatile and less error prone
> for function tracing.
> 
> Starting with v4.19, the minimum gcc supported to build the Linux kernel,
> was raised to version 4.6. That also happens to be the first gcc version to
> support -mfentry. Since on x86, using gcc versions from 4.6 and beyond will
> unconditionally enable the -mfentry, it will no longer use mcount as the
> method for inserting calls into the C functions of the kernel. This means
> that there is no point in continuing to maintain mcount in x86.
> 
> Remove support for using mcount. This makes the code less complex, and will
> also allow it to be simplified in the future.
> 
> Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>

Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>