[libre-riscv-dev] LibreSOC - RISCV and POWER dual architecture feasibility

[Hendrik Boom]

On Sat, Mar 14, 2020 at 08:17:07PM -0700, Jacob Lifshay wrote:
> On Sat, Mar 14, 2020, 18:01 Immanuel, Yehowshua U <yimmanuel3 at gatech.edu>
> wrote:
> 
> > Hello Jacob,
> >         I’m running through the numbers and studying LibreSOC’s potential
> > markets, and how we’d market the dual architecture.
> > Just how feasible is the dual architecture we’re currently embarking on.
> > Luke says its doable.
> > What are your thoughts, and what are the challenges - just trying to get a
> > comprehensive understanding.
> >
> 
> It will be quite easy to do from the hardware side due to the relative
> simplicity of user-mode RV64GC -- basically all we need is to do is
> implement the instruction decoder and clean up the odds and ends that
> result from the semantic differences.
> 
> The software side will be a little harder but still relatively easy, we
> need to implement support for translating Linux system calls from the
> RISC-V interface to the internal calls, as well as implementing the
> syscalls for switching modes between ISAs -- that's like 80% of the work on
> the Linux Kernel side. For userspace, we can have just executing RISC-V ELF
> executables as a MVP, later, we can implement the stuff in the dynamic
> linker to handle having both Power and RV code loaded in the same process
> -- I'm thinking we should use a system similar to Wine and Darling (like
> Wine but macOS on Linux) where they have two dynamic linkers in the same
> process. Will have to figure that out.

Is the multiarch feature of Linux of any use here?  I think it 
allows a choice of architecture for each *process*.  So we'd 
just have to have two sets of libraries.  We wouldn't have to link 
any code that contains both architectures.

Architecture switch would happen at system call and at process 
switch.

-- hendrik 
> 
> We can also build a bare-metal hypervisor-like program to emulate RV system
> mode to allow easier testing (don't need to boot a whole Linux kernel) as
> well as booting a whole RISC-V OS. I'd estimate that it would be less than
> 10kloc of code.
> 
> Jacob
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