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    SubjectRe: [patch 00/38] x86/retbleed: Call depth tracking mitigation
    On Mon, Jul 18, 2022 at 12:30 PM Thomas Gleixner <tglx@linutronix.de> wrote:
    >
    > Let the compiler add a 16 byte padding in front of each function entry
    > point and put the call depth accounting there. That avoids calling out
    > into the module area and reduces ITLB pressure.

    Ooh.

    I actually like this a lot better.

    Could we just say "use this instead if you have SKL and care about the issue?"

    I don't hate your module thunk trick, but this does seem *so* much
    simpler, and if it performs better anyway, it really does seem like
    the better approach.

    And people and distros who care would have an easy time adding that
    simple compiler patch instead.

    I do think that for generality, the "-mforce-function-padding" option
    should perhaps take as an argument how much padding (and how much
    alignment) to force:

    -mforce-function-padding=5:16

    would force 5 bytes of minimum padding, and align functions to 16
    bytes. It should be easy to generate (no more complexity than your
    current one) by just making the output do

    .skip 5,0xcc
    .palign 4,0xcc

    and now you can specify that you only need X bytes of padding, for example.

    Linus

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