On Thu, 2 Jun 2022, Vedvyas Shanbhogue via lists.riscv.org wrote:

Based on discussion it did not seem like it needs to be much fancier than
this as this is for early OS/VMM code till it has enough functionality to
directly interact with a uart.

The goal of the shared memory based SBI call for early prints is to
minimize the number of traps which in-turn helps virtualization to
drastically reduce boot-time.

Understand that better now. But if that is the main motivation then
I am not understanding why we would want to push all of this into
M-mode firmware vs. defining a set of standardized pv-ops to be used
by guest OSes.

That is because it is useful to have debug console output when porting a
hypervisor or baremetal code to a new board.

Of course, if a hypervisor is already available for the board, then it
would be just as easy to use a paravirtualized interface, e.g. Xen's
HYPERVISOR_console_io hypercall. But somebody has to port the
hypervisor first :-)