Researchers from Google investigating the scope and impact of the Spectre attack have published a paper asserting that Spectre-like vulnerabilities are likely to be a continued feature of processors and, further, that software-based techniques for protecting against them will both impose a high performance cost. In any case, the researchers continue, the software will be inadequate—some Spectre flaws don't appear to have any effective software-based defense. As such, Spectre is going to be a continued feature of the computing landscape, with no straightforward resolution.
The discovery and development of the Meltdown and Spectre attacks was undoubtedly the big security story of 2018. First revealed last January, new variants and related discoveries were made throughout the rest of the year. Both attacks rely on discrepancies between the theoretical architectural behavior of a processor—the documented behavior that programmers depend on and write their programs against—and the real behavior of implementations.
Specifically, modern processors all perform speculative execution; they make assumptions about, for example, a value being read from memory or whether an if condition is true or false, and they allow their execution to run ahead based on these assumptions. If the assumptions are correct, the speculated results are kept; if it isn't, the speculated results are discarded and the processor redoes the calculation. Speculative execution is not an architectural feature of the processor; it's a feature of implementations, and so it's supposed to be entirely invisible to running programs. When the processor discards the bad speculation, it should be as if the speculation never even happened.