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Re: [Qemu-riscv] [Qemu-devel] RISC-V: Vector && DSP Extension

From: Palmer Dabbelt
Subject: Re: [Qemu-riscv] [Qemu-devel] RISC-V: Vector && DSP Extension
Date: Tue, 27 Aug 2019 17:25:40 -0700 (PDT)

On Thu, 22 Aug 2019 15:37:15 PDT (-0700), address@hidden wrote:
On Wed, Aug 21, 2019 at 6:56 PM liuzhiwei <address@hidden> wrote:

On 2019/8/22 上午3:31, Palmer Dabbelt wrote:
> On Thu, 15 Aug 2019 14:37:52 PDT (-0700), address@hidden wrote:
>> On Thu, Aug 15, 2019 at 2:07 AM Peter Maydell
>> <address@hidden> wrote:
>>> On Thu, 15 Aug 2019 at 09:53, Aleksandar Markovic
>>> <address@hidden> wrote:
>>> >
>>> > > We can accept draft
>>> > > extensions in QEMU as long as they are disabled by default.
>>> > Hi, Alistair, Palmer,
>>> >
>>> > Is this an official stance of QEMU community, or perhaps Alistair's
>>> > personal judgement, or maybe a rule within risv subcomunity?
>>> Alistair asked on a previous thread; my view was:
>>> https://lists.gnu.org/archive/html/qemu-devel/2019-07/msg03364.html
>>> and nobody else spoke up disagreeing (summary: should at least be
>>> disabled-by-default and only enabled by setting an explicit
>>> property whose name should start with the 'x-' prefix).
>> Agreed!
>>> In general QEMU does sometimes introduce experimental extensions
>>> (we've had them in the block layer, for example) and so the 'x-'
>>> property to enable them is a reasonably established convention.
>>> I think it's a reasonable compromise to allow this sort of work
>>> to start and not have to live out-of-tree for a long time, without
>>> confusing users or getting into a situation where some QEMU
>>> versions behave differently or to obsolete drafts of a spec
>>> without it being clear from the command line that experimental
>>> extensions are being enabled.
>>> There is also an element of "submaintainer judgement" to be applied
>>> here -- upstream is probably not the place for a draft extension
>>> to be implemented if it is:
>>>  * still fast moving or subject to major changes of design direction
>>>  * major changes to the codebase (especially if it requires
>>>    changes to core code) that might later need to be redone
>>>    entirely differently
>>>  * still experimental
>> Yep, agreed. For RISC-V I think this would extend to only allowing
>> extensions that have backing from the foundation and are under active
>> discussion.
> My general philosophy here is that we'll take anything written down in
> an official RISC-V ISA manual (ie, the ones actually released by the
> foundation).  This provides a single source of truth for what an
> extension name / version means, which is important to avoid
> confusion.  If it's a ratified extension then I see no reason not to
> support it on my end.  For frozen extensions we should probably just
> wait the 45 days until they go up for a ratification vote, but I'd be
> happy to start reviewing patches then (or earlier :)).
> If the spec is a draft in the ISA manual then we need to worry about
> the support burden, which I don't have a fixed criteria for --
> generally there shouldn't be issues here, but early drafts can be in a
> state where they're going to change extensively and are unlikely to be
> used by anyone.  There's also the question of "what is an official
> release of a draft specification?".
> That's a bit awkward right now: the current ratified ISA manual
> contains version 0.3 of the hypervisor extension, but I just talked to
> Andrew and the plan is to remove the draft extensions from the
> ratified manuals because these drafts are old and the official manuals
> update slowly.  For now I guess we'll need an an-hoc way of
> determining if a draft extension has been officially versioned or not,
> which is a bit of a headache.
> We already have examples of supporting draft extensions, including
> priv-1.9.1.  This does cause some pain for us on the QEMU side (CSR
> bits have different semantics between the specs), but there's 1.9.1
> hardware out there and the port continues to be useful so I'd be in
> favor of keeping it around for now.  I suppose there is an implicit
> risk that draft extensions will be deprecated, but the "x-" prefix,
> draft status, and long deprecation period should be sufficient to
> inform users of the risk.  I wouldn't be opposed to adding a "this is
> a draft ISA" warning, but I feel like it might be a bit overkill.
Hi, Palmer

Maybe it is the headache of open source hardware. Everyone cooperates to
build a better architecture.

In my opinion, we should focus on the future. The code in QEMU mainline
should evolve to the  ratified extension step by step, and only support
the best extension at last.

At that time,  even many hardwares just support  the deprecated draft
extension,  the draft codes should be in the wild and maintained by the
hardware manufactures.

But before that,  it is better to  have a draft implementation. So that
We can work step by step to accelerate the coming of the ratified

Even at last draft extension implementation are deprecated, they are not
meaningless. The manufactures may use  the  history commit to support
their hardwares that

only support drafted extension.

Overall I agree with Palmer that we should accept all extensions in
the RISC-V foundations ISA manual once the extension has reached some
level of maturity.

I think it then makes sense to only keep the latest version of these
drafts until they are ratified. At that point we do have to support
the ratified version for longer.

My number one constraint here is that QEMU remains a useful tool. I'd love to support every version of every extension ever published (along with allowing for control of all the optional features), but there are just more important things to do so that's not going to happen. That said, I also want to make sure that there is some stability in QEMU -- specifically because having a RISC-V implementation that supports multiple targets is very useful. I started hacking on QEMU because I wanted to test my other software and it's just too cumbersome to keep around a pile of RISC-V implementations to test everything against.

In terms of which drafts we keep, I think it's best to just play this by ear -- for example, I'd be in favor of keeping draft extensions around if they end up widely implemented, and doubly so if they're similar to the ratified extension. The CLIC would be a good candidate for this: it's in all sorts of hardware, but the actual ratified spec may be a bit different than the drafts. In that case I'd be in favor of keeping around the draft specs for a long time, as they're probably going to be pretty easy to support.


Best Regards,


>> Alistair
>>> thanks
>>> -- PMM

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