eigenmodes

Does anyone know how to modify Bathe's subspace iteration eigensolver to compute the highest eigenvalue instead of the smallest ?

I want to use Intel Math Kernel Library (MKL) to calculate the bifurcation loads and modes in my custom FE program. MKL has many algorithms and I want to use the most efficient one.
Basically, I need to find a few eigenvalues lambda_i and eigenvectors u_i for the problem
KG * u = lambda * KS * u
where KG is the stiffness matrix and KS is the geometric stiffness matrix, also called stress-displacement matrix.
Is KG supposed to be positive definite?