Creating SSRS Reports on user login with data source as SSAS cubes

I want to create SSRS reports where my data source is SSAS cubes and these reports will have the filtered data based on the user who has logged in.
My requirement is that : I do have a mapping table where I have mapped the username with the userid.
My fact table contains the userid's of various persons and the respective data allocated to them.

1)How should I capture the username of the user who has logged in to the reports and pass it to the SSAS cubes?
2)How to filter the data from the cubes based on the user login from cubes?


Its urgent. Pls help