An industry-wide issue was found in the way many modern microprocessor designs have implemented speculative execution of instructions (a commonly used performance optimization). There are three primary variants of the issue which differ in the way the speculative execution can be exploited.
Variant CVE-2017-5715 triggers the speculative execution by utilizing branch target injection. It relies on the presence of a precisely-defined instruction sequence in the privileged code as well as the fact that memory accesses may cause allocation into the microprocessor's data cache even for speculatively executed instructions that never actually commit (retire). As a result, an unprivileged attacker could use this flaw to cross the syscall and guest/host boundaries and read privileged memory by conducting targeted cache side-channel attacks.
Converting into tracker bug, needed for Xen as well.
Just linking a post by David Woodhouse here, as is greatly clarifies the various mitigation techiques that have been devised. An important aspect of the present situation is that retpolines, alone, constitute a viable mitigation technique for CVE-2017-5715 - but not wholly so if using Skylake or newer.