Just to clarify what you mean by direct: Do you mean direct as in supplying troops to fight in the south or indirect as in providing North Vietnam with weapons, training, advisors, etc?

Possible chinese/soviet personell manning air defence sites was a major reason why the bombing of the north was so restricted and micro managed by the president and secretary of defence for fear of killing them and spreading the scope of the war. The same with mining Haiphong harbour.