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

From: Peter Maydell <peter.maydell@linaro.org>
To: Aleksandar Markovic <aleksandar.m.mail@gmail.com>
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>,
	Alistair Francis <alistair23@gmail.com>,
	LIU ZhiWei <zhiwei_liu@c-sky.com>
Subject: Re: [Qemu-devel] RISC-V: Vector && DSP Extension
Date: Thu, 15 Aug 2019 10:07:45 +0100	[thread overview]
Message-ID: <CAFEAcA9bDU4AWetkR4w4WLUzMW_MMy0CsVa5SibWqf85RyaX5g@mail.gmail.com> (raw)
In-Reply-To: <CAL1e-=hVLQWThYSm78bTCbQPfAABjx_rG9nEa9gV6PvSN8ge2w@mail.gmail.com>

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).

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

thanks
-- PMM