[RFC][PATCH] objtool,x86_64: Replace recordmcount with objtool

[Peter Zijlstra]

On Wed, Jul 22, 2020 at 01:55:42PM -0400, Steven Rostedt wrote:

> > Ha! it is trying to convert the "CALL __fentry__" into a NOP and not
> > finding the CALL -- because objtool already made it a NOP...
> > 
> > Weird, I thought recordmcount would also write NOPs, it certainly has
> > code for that. I suppose we can use CC_USING_NOP_MCOUNT to avoid those,
> > but I'd rather Steve explain this before I wreck things further.
> 
> The reason for not having recordmcount insert all the nops, is because
> x86 has more than one optimal nop which is determined by the machine it
> runs on, and not at compile time. So we figured just updated it then.
> 
> We can change it to be a nop on boot, and just modify it if it's not
> the optimal nop already. 

Right, I throught that's what we'd be doing already, anyway:

> That said, Andi Kleen added an option to gcc called -mnop-mcount which
> will have gcc do both create the mcount section and convert the calls
> into nops. When doing so, it defines CC_USING_NOP_MCOUNT which will
> tell ftrace to expect the calls to already be converted.

That seems like the much easier solution, then we can forget about
recordmcount / objtool entirely for this.