Re: [Xen-devel] [RFC XEN PATCH 00/23] xen: beginning support for RISC-V

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 22/01/2020 01:58, Bobby Eshleman wrote:
> Hey everybody,
>
> This is an RFC patchset for the very beginnings of adding RISC-V support
> to Xen.  This RFC is really just to start a dialogue about supporting
> RISC-V and align with the Xen project and community before going
> further.  For that reason, it is very rough and very incomplete. 
>
> My name is Bobby Eshleman, I'm a software engineer at
> Star Lab / Wind River on the ARM team, mostly having worked in the Linux
> kernel.  I've also been involved a good amount with Xen on ARM here,
> mostly dealing with tooling, deployment, and testing.  A lot of this
> patchset is heavily inspired by the Xen/ARM source code (particularly
> the early setup up code).
>
> Currently, this patchset really only sets up virtual memory for Xen and
> initializes UART to enable print output.  None of RISC-V's
> virtualization support has been implemented yet, although that is the
> next road to start going down.  Many functions only contain dummy
> implementations.  Many shortcuts have been taken and TODO's have been
> left accordingly.  It is very, very rough.  Be forewarned: you are quite
> likely to see some ungainly code here (despite my efforts to clean it up
> before sending this patchset out).  My intent with this RFC is to align
> early and gauge interest, as opposed to presenting a totally complete
> patchset.
>
> Because the ARM and RISC-V use cases will likely bear resemblance, the
> RISC-V port should probably respect the design considerations that have
> been laid out and respected by Xen on ARM for dom0less, safety
> certification, etc...  My inclination has been to initially target or
> prioritize dom0less (without excluding dom0full) and use the ARM
> dom0less implementation as a model to follow.  I'd love feedback on this
> point and on how the Xen project might envision a RISC-V implementation.
>
> This patchset has _some_ code for future support for 32-bit, but
> currently my focus is on 64-bit.
>
> Again, this is a very, very rough and totally incomplete patchset.  My
> goal here is just to gauge community interest, begin discussing what Xen
> on RISC-V may look like, receive feedback, and see if I'm heading in the
> right direction.
>
> My big questions are:
>       Does the Xen project have interest in RISC-V?

There is very large downstream interest in RISC-V.  So a definite yes.

>       What can be done to make the RISC-V port as upstreamable as
>               possible?
>       Any major pitfalls?
>
> It would be great to hear all of your feedback.

Both RISC-V and Power9 are frequently requested things, and both suffer
from the fact that, while we as a community would like them, the
upstream intersection of "people who know Xen" and "people who know
enough arch $X to do an initial port" is 0.

This series clearly demonstrates a change in the status quo, and I think
a lot of people will be happy.

To get RISC-V to being fully supported, we will ultimately need to get
hardware into the CI system, and an easy way for developers to test
changes.  Do you have any thoughts on production RISC-V hardware
(ideally server form factor) for the CI system, and/or dev boards which
might be available fairly cheaply?

How much time do you have to put towards the port?  Is this something in
your free time, or something you are doing as part of work?  Ultimately,
we are going to need to increase the level of RISC-V knowledge in the
community to maintain things in the future.

Other than that, very RFC series are entirely fine.  A good first step
would be simply to get the build working, and get some kind of
cross-compile build in CI, to make sure that we don't clobber the RISC-V
build with common or other-arch changes.

I hope this helps.

~Andrew

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