The AVX documentation says that all VEX prefixes require ymm state saving support to be enabled. Shouldn't only the ones that premote the operation to 256-bit require ymm state saving support?
VEX prefix has both 128-bit and 256-bit instructions, so most of SIMD (SSE2, SSE3, SSE4x) instructions are promoted to use VEX prefix. if you are using any VEX instruction then you have to enable YMM State (VEX promoted instructions and new VEX instructions). Because then HW maintains 256-bit state of the registers, even if you are using 128bit VEX instructions. if your code is not using VEX instructions or AVX instructions then you dont need to enable the YMM State.