TurretinFan has posted: Calvin and the Perpetual Virginity (?) of Mary ... plus the Real Francis Turretin. Keep it bookmarked, because Calvin is often brought forth as a witness for Roman Catholic Mariology.
I've done a few links on this topic as well:
On John Calvin's "Mariology"
John Calvin on the Greatness of Mary
John Calvin on Latria and Dulia
Bibliographic Tedium on the Reformers and Perpetual Virginity
The closest I recall coming to actual information about Calvin holding positively to Mary's perpetual virginity was in a discussion with Paul Hoffer in a comment box on this blog. If I recall, he had a secondary source in French with Calvin quotes, but no context. That discussion is somewhere buried on the blog.