Ah, I found the recent change about XSAVE: it is a fix for CVE-2018-3665 vulnerability.

"The software mitigation for this is to switch to an "eager" / immediate FPU state save and restore, in both kernels and hypervisors."

"On Intel and AMD x86 processors, operating systems and hypervisors often use what is referred to as a deferred saving and restoring method of the x86 FPU state, as part of performance optimization. This is done in a "lazy" on-demand fashion."

"It was found that due to the "lazy" approach, the x86 FPU states or FPU / XMM / AVX512 register content, could leak across process, or even VM boundaries, giving attackers possibilities to read private data from other processes, when using speculative execution side channel gadgets."

https://www.suse.com/support/kb/doc/?id=7023076

See also: https://en.wikipedia.org/wiki/Lazy_FP_state_restore