  The numerical study of relativistic magnetohydrodynamics (MHD) plays a
crucial role in high-energy astrophysics, but unfortunately is computationally
demanding, given the complex physics involved (high Lorentz factor flows,
extreme magnetization, curved spacetimes near compact objects) and the large
variety of spatial scales needed to resolve turbulent motions. A great benefit
comes from the porting of existing codes running on standard processors to
GPU-based platforms. However, this usually requires a drastic rewriting of the
original code, the use of specific languages like CUDA, and a complex analysis
of data management and optimization of parallel processes. Here we describe the
porting of the ECHO code for special and general relativistic MHD to
accelerated devices, simply based on native Fortran language built-in
constructs, especially 'do concurrent' loops, few OpenACC directives, and the
straightforward data management provided by the Unified Memory option of NVIDIA
compilers.Thanks to these very minor modifications to the original code, the
new version of ECHO runs at least 16 times faster on GPU platforms compared to
CPU-based ones. The chosen benchmark is the 3D propagation of a relativistic
MHD Alfv\'en wave, for which strong and weak scaling tests performed on the
LEONARDO pre-exascale supercomputer at CINECA are provided (using up to 256
nodes corresponding to 1024 GPUs, and over 14 billion cells). Finally, an
example of high-resolution relativistic MHD Alfv\'enic turbulence simulation is
shown, demonstrating the potential for astrophysical plasmas of the new
GPU-based version of ECHO.
