Intel volume 2, section LDMXCSR wrote:
Note: In VEX-encoded versions, VEX.vvvv is reserved and must be 1111b, otherwise instructions will #UD.
Intel LDMXCSR: Exceptions wrote:
#UD If VEX.vvvv ≠ 1111B.
also, Intel manual volume 2, 2.4.5 Exceptions Type 5 includes a table (labeled table 2-22 in my latest edition) gives a list of causes of various causes for exceptions in various modes
in #UD in RMode for type 5 exception-class can be caused by:
-VEX prefix
-legacy SSE instructions if CR0.EM=1 or CR4.OSFXSR=0
-if preceded by lock instruction
-if any corresponding CPUID feature flag is 0
there is nothing in the "ldmxcsr" section that says it only works in PMode