OpenGL vs DirectX attribute buffers

After over a decade in OpenGL I finally poked at some DirectX. I'm curious how the driver deals with this if anyone knows. Might be confusing but the basics are my mesh data below doesn't have a w coordinate for obvious data reasons. In DirectX I can simply bind the position data as a float4, and just set the w coordinate. In OpenGL I am not allowed to modify the w coordinate of an input. I have to create a temporary vec4 from the input and set w to 1.

My assumption here is that in Directx the …