  Fueling and feedback couple supermassive black holes (SMBHs) to their host
galaxies across many orders of magnitude in spatial and temporal scales, making
this problem notoriously challenging to simulate. We use a multi-zone
computational method based on the general relativistic magneto-hydrodynamic
(GRMHD) code KHARMA that allows us to span $7$ orders of magnitude in spatial
scale, to simulate accretion onto a non-spinning SMBH from an external medium
with Bondi radius $R_B\approx 2\times 10^5 \,GM_\bullet/c^2$, where $M_\bullet$
is the SMBH mass. For the classic idealized Bondi problem, spherical gas
accretion without magnetic fields, our simulation results agree very well with
the general relativistic analytic solution. Meanwhile, when the accreting gas
is magnetized, the SMBH magnetosphere becomes saturated with a strong magnetic
field. The density profile varies as $\sim r^{-1}$ rather than $r^{-3/2}$ and
the accretion rate $\dot{M}$ is consequently suppressed by over 2 orders of
magnitude below the Bondi rate $\dot{M}_B$. We find continuous energy feedback
from the accretion flow to the external medium at a level of
$\sim10^{-2}\dot{M}c^2 \sim 5\times 10^{-5} \dot{M}_B c^2$. Energy transport
across these widely disparate scales occurs via turbulent convection triggered
by magnetic field reconnection near the SMBH. Thus, strong magnetic fields that
accumulate on horizon scales transform the flow dynamics far from the SMBH and
naturally explain observed extremely low accretion rates compared to the Bondi
rate, as well as at least part of the energy feedback.
