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    SubjectRe: [PATCH 1/1] x86/speculation: Allow IBPB to be conditionally enabled on CPUs with always-on STIBP
    On Tue, Nov 03, 2020 at 11:57:57AM +0100, Borislav Petkov wrote:
    > On Mon, Nov 02, 2020 at 11:02:10AM +1100, Anand K. Mistry wrote:
    > > > I like the idea of passing in the mode you want to check, but it appears
    > > > they are never used independently. The ibpb and stibp modes are always
    > > > checked together in one of the if statements below, so you could make this
    > > > a function that checks both modes and just have a single call. I'll leave
    > > > that up to the maintainers to see what is preferred.
    > >
    > > I can see both sides to this. Personally, I think I prefer it as-is
    > > since I think it improves readability a bit by making the conditions
    > > less complicated whilst not hiding too many details. I'll wait to see
    > > what others say before changing this one.
    >
    > Yes, but if you make it a single function with a descriptive name, you'd
    > make the call sites even more readable:
    >
    > if (!is_spec_ib_conditional(..))
    > bla;
    >
    > or
    >
    > if (!is_spec_ib_user_controlled(..))
    > blu;
    >
    > and that function should simply check both spectre_v2_user_ibpb *and*
    > spectre_v2_user_stibp in one go.
    >
    > Why should we do that?
    >
    > Exactly because you both got your brains twisted just from looking at
    > this. Because this mitigation crap is such an ugly and complex maze that
    > we would take even the smallest simplification any day of the week!
    >
    > Welcome to my life since meltdown. Brain twist feels good, doesn't it?
    >
    > :-)))

    I hate the maze too. In theory we can get rid of STIBP if/when
    core-scheduling is enabled because the cross-CPU branch predictor poisioning
    would not be possible. Maybe that will simplify the maze a bit.

    thanks,

    - Joel

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