As is known to all.是否为从句,若是从句,那么此从句中的主语又是什么

As is known to all,是非限制定语从句,在从句中 As 作主语,例如：
As is known to all,Taiwan belongs to China.众所周知,台湾属于中国.
(As 在句中表示Taiwan belongs to China.整个句子 )
as 引导定语从句时,位置比较灵活,可放句首,可放句末,还可以插在句中.
as 还可以在从句中作主语,例如：
As you know ,Taiwan belongs to China.（as 在此句中做 know 的宾语,同时表示整个主句内容）