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

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