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SubjectRe: [PATCH v16 00/17] KVM RISC-V Support
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On Wed, 31 Mar 2021 02:21:58 PDT (-0700), pbonzini@redhat.com wrote:
> On 30/03/21 07:48, Anup Patel wrote:
>>
>> It seems Andrew does not want to freeze H-extension until we have virtualization
>> aware interrupt controller (such as RISC-V AIA specification) and IOMMU. Lot
>> of us feel that these things can be done independently because RISC-V
>> H-extension already has provisions for external interrupt controller with
>> virtualization support.

Sorry to hear that. It's really gotten to a point where I'm just
embarrassed with how the RISC-V foundation is being run -- not sure if
these other ones bled into Linux land, but this is the third ISA
extension that's blown up over the last few weeks. We had a lot of
discussion about this on the binutils/GCC side of things and I've
managed to convince myself that coupling the software stack to the
specification process isn't viable -- we made that decision under the
assumption that specifications would actually progress through the
process, but in practice that's just not happening.

My goal with the RISC-V stuff has always been getting us to a place
where we have real shipping products running a software stack that is as
close as possible to the upstream codebases. I see that as the only way
to get the software stack to a point where it can be sustainably
maintained. The "only frozen extensions" policy was meant to help this
by steering vendors towards a common base we could support, but in
practice it's just not working out. The specification process is just
so unreliable that in practice everything that gets built ends up
relying on some non-standard behavior: whether it's a draft extension,
some vendor-specific extension, or just some implementation quirks.
There's always going to be some degree of that going on, but over the
last year or so we've just stopped progressing.

My worry with accepting the draft extensions is that we have no
guarantee of compatibility between various drafts, which makes
supporting multiple versions much more difficult. I've always really
only been worried about supporting what gets implemented in a chip I can
actually run code on, as I can at least guarantee that doesn't change.
In practice that really has nothing to do with the specification freeze:
even ratified specifications change in ways that break compatibility so
we need to support multiple versions anyway. That's why we've got
things like the K210 support (which doesn't quite follow the ratified
specs) and are going to take the errata stuff. I hadn't been all that
worried about the H support because there was a plan to get is to
hardware, but with the change I'm not really sure how that's going to
happen.

> Yes, frankly that's pretty ridiculous as it's perfectly possible to
> emulate the interrupt controller in software (and an IOMMU is not needed
> at all if you are okay with emulated or paravirtualized devices---which
> is almost always the case except for partitioning hypervisors).

There's certainly some risk to freezing the H extension before we have
all flavors of systems up and running. I spent a lot of time arguing
that case years ago before we started telling people that the H
extension just needed implementation, but that's not the decision we
made. I don't really do RISC-V foundation stuff any more so I don't
know why this changed, but it's just too late. It would be wonderful to
have an implementation of everything we need to build out one of these
complex systems, but I just just don't see how the current plan gets
there: that's a huge amount of work and I don't see why anyone would
commit to that when they can't count on it being supported when it's
released.

There are clearly some systems that can be built with this as it stands.
They're not going to satisfy every use case, but at least we'll get
people to start seriously using the spec. That's the only way I can see
to move forward with this. It's pretty clear that sitting around and
waiting doesn't work, we've tried that.

> Palmer, are you okay with merging RISC-V KVM? Or should we place it in
> drivers/staging/riscv/kvm?

I'm certainly ready to drop my objections to merging the code based on
it targeting a draft extension, but at a bare minimum I want to get a
new policy in place that everyone can agree to for merging code. I've
tried to draft up a new policy a handful of times this week, but I'm not
really quite sure how to go about this: ultimately trying to build
stable interfaces around an unstable ISA is just a losing battle. I've
got a bunch of stuff going on right now, but I'll try to find some time
to actually sit down and finish one.

I know it might seem odd to complain about how slowly things are going
and then throw up another roadblock, but I really do think this is a
very important thing to get right. I'm just not sure how we're going to
get anywhere with RISC-V without someone providing stability, so I want
to make sure that whatever we do here can be done reliably. If we don't
I'm worried the vendors are just going to go off and do their own
software stacks, which will make getting everyone back on the same page
very difficult.

> Either way, the best way to do it would be like this:
>
> 1) you apply patch 1 in a topic branch
>
> 2) you merge the topic branch in the risc-v tree
>
> 3) Anup merges the topic branch too and sends me a pull request.
>
> Paolo

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