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

From: Alistair Francis <alistair23@gmail.com>
To: Peter Maydell <peter.maydell@linaro.org>
Cc: "open list:RISC-V" <qemu-riscv@nongnu.org>,
	Sagar Karandikar <sagark@eecs.berkeley.edu>,
	bastian@mail.uni-paderborn.de, Palmer Dabbelt <palmer@sifive.com>,
	"qemu-devel@nongnu.org Developers" <qemu-devel@nongnu.org>,
	Alistair Francis <Alistair.Francis@wdc.com>,
	LIU ZhiWei <zhiwei_liu@c-sky.com>,
	Aleksandar Markovic <aleksandar.m.mail@gmail.com>
Subject: Re: [Qemu-devel] RISC-V: Vector && DSP Extension
Date: Thu, 15 Aug 2019 14:37:52 -0700	[thread overview]
Message-ID: <CAKmqyKM44ZAHgc5cYAiAXXtVG=dMcX1i7FLn+2mMwM1Av4Gqzg@mail.gmail.com> (raw)
In-Reply-To: <CAFEAcA9bDU4AWetkR4w4WLUzMW_MMy0CsVa5SibWqf85RyaX5g@mail.gmail.com>

On Thu, Aug 15, 2019 at 2:07 AM Peter Maydell <peter.maydell@linaro.org> wrote:
>
> On Thu, 15 Aug 2019 at 09:53, Aleksandar Markovic
> <aleksandar.m.mail@gmail.com> 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.

Alistair

>
> thanks
> -- PMM