Yes, all software changes should be tested

OK, I apologize. I was only going by what I had read on LWN and other sources. At the time, the address space separation was merged into the kernel but there was no mention of whether it mitigated the Meltdown attack. (The details of Meltdown only became public a few days afterwards.) As far as an outsider could tell, it was just merged because address space separation was generally thought to mitigate some theoretical attack that was likely to be possible, but without any test cases for a specific attack.