Address Collision using i386 4G/4G Memory Split

[Konstantin Belousov]

On Mon, Dec 17, 2018 at 02:51:48PM +0100, Alexander Lochmann wrote:
> Hi folks!
> 
> According to git commit e3089a (https://reviews.freebsd.org/D1463)
> FreeBSD 12.0 i386 uses separate address spaces for kernel and user
> space. So basically two memory areas, one in each space, can have the
> same address.
> Is this possible with FreeBSD 12.0? Is this likely to happen?
The feature was added to HEAD during this summer, before stable/12 was
branched.

> 
> On my opinion, this is also very expensive in terms of performance.
> Any copy{in,out} has to flush the TLB.
> (http://fxr.watson.org/fxr/source/i386/i386/copyout_fast.s#L91)
> Why are you still using this 4G/4G approach?
Because it is needed for i386 to self-host, in modern world 1G KVA
is too small, and because it provides Meltdown mitigation.