The old barrier implementation was very slow when running on a multi-socket
machine (pcmemtest issue 16).
The new implementation provides two options:
- when blocked, spin on a thread-local flag
- when blocked, execute a HLT instruction and wait for a NMI
The first option might be faster, but we need to measure it to find out. A
new boot command line option is provided to select between the two, with a
third setting that uses a mixture of the two.
When using a legacy BIOS, the memory regions used by the BIOS are well
defined. This is not the case when using a UEFI BIOS. So include the
stack area in the BSS so the loader knows how much memory to allocate,
and check we have space to relocate the program to either low or high
memory.
There are still some assumptions in the USB driver code that need to
be fixed.
