* Improve abstraction in vmem.h and limit memory benchmarking to first 2GB.
The third GB may get used for remapping memory regions that are only
accessed during startup, so it's not safe to use it for the memory
speed tests.
* Fix calculation of end limit for locating memory benchmark workspace.
* Document vmem.h.
* Use window number, not current start address, to detect first window.
* Increase the program low-load range from 1MB to 4MB and make more robust.
If the BIOS has reserved some parts of low memory, there may not be
enough contiguous space left to load the program there (issue #49).
So increase the low-load range to include the first 3MB of high
memory. Also guard against the program being initially loaded
straddling the new boundary.
Co-authored-by: Martin Whitaker <memtest@martin-whitaker.me.uk>
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.
Mostly we write and read large chunks of data which will make it likely
that the data is no longer in the cache when we come to verify it. But
this is not always true, and in any case, we shouldn't rely on it.
