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>> On processors with enhanced IBRS support, we recommend setting IBRS to 1
>> and left set.
> 
> Then why doesn't CPU with EIBRS support acutally *default* to '1', with
> opt-out possibility for OS?

(slightly longer answer)

you can pretty much assume that on these CPUs, IBRS doesn't actually do anything
(e.g. just a scratch bit)

we could debate (and did :-)) for some time what the default value should be at boot,
but it kind of is one of those minor issues that should not hold up getting things out.

it could well be that the cpus that do this will ship with 1 as default, but it's hard to
guarantee across many products and different CPU vendors when time was tight.