Address Collision using i386 4G/4G Memory Split

Kurt Lidl lidl at
Mon Dec 17 20:58:14 UTC 2018

Alexander Lochmann writes:
> According to git commit e3089a (
> 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?

If the userspace program and the kernel address happen to overlap, the 
system will deal with it.  There's not anything to worry about.  As to
whether or not it's likely to happen -- I'm not sure about that.  I
expect the default stack and heap space locations for a fresh process
have changed due to this change, but it should not matter.

> On my opinion, this is also very expensive in terms of performance.
> Any copy{in,out} has to flush the TLB.
> (
> Why are you still using this 4G/4G approach?

The complete split between the user address space and kernel address 
space mapping is largely due to the mitigation of the Spectre attacks,
as I understand things.  To have both the kernel and userspace mapped
at the same time, can be used to extract information from the kernel
that should not be made available.

I think it falls into the "slower but safer" class of change.

Someone will, undoubtedly, correct me if I'm wrong.


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