PETSC_EXTERN PetscErrorCode VecCUDAGetArrayRead(Vec v, const PetscScalar **a)This function is analogous to VecGetArrayRead(): The pointer returned by this function points to a consistent view of the vector data. This may involve a copy operation of data from the host to the device if the data on the device is out of date. If the device memory hasn't been allocated previously it will be allocated as part of this function call. VecCUDAGetArrayRead() assumes that the user will not modify the vector data. This is analgogous to intent(in) in Fortran.
The CUDA device pointer has to be released by calling VecCUDARestoreArrayRead(). If the data on the host side was previously up to date it will remain so, i.e. data on both the device and the host is up to date. Accessing data on the host side does not incur a device to host data transfer.