What is the grammatical role of al-qalba in mahmā taʾmurī al-qalba yafʿal?

Imruʾ al-Qays: Address to Fāṭimah, Parting, and the Arrows of the Eyes (Verses 19–22)

• The subject of mahmā
• A preposition attached to yafʿal
• The nominative noun of kāna
• The direct object of taʾmurī