Got it, the principle is once XCR0[n]=1 and XFD[n]=0, then guest is allowed
to use the dynamic XSAVE state, thus KVM must prepare all things well
before. This probably happens shortly after guest #NM.
Only one thing: it seems we assume that vcpu->arch.xfd is guest runtime
value. And before guest initializes XFD, KVM provides
vcpu->arch.xfd[18]=1, right? But the spec asks XFD reset value as zero.
If so, between guest init XCR0 to 1 and init XFD to 1, it's XCR0[n]=1 and
XFD[n]=0. If a guest never init XFD and directly use dynamic state...
Or do we want to provide guest a XFD[18]=1 value at the very beginning?