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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Wed, 7 Nov 2018 10:51:11 +0100
Message-ID: <CAK8P3a1He1B2UHLEinzuPF2WvDbt=gwmTkz7-XoMwvpHhBDiuA@mail.gmail.com>
Subject: Re: [RFC 0/2] RISC-V: A proposal to add vendor-specific code
To:     Palmer Dabbelt <palmer@sifive.com>
Cc:     Christoph Hellwig <hch@infradead.org>, vincentc@andestech.com,
        aou@eecs.berkeley.edu, alankao@andestech.com,
        greentime@andestech.com, linux-kernel@vger.kernel.org,
        zong@andestech.com, kito@andestech.com,
        linux-riscv@lists.infradead.org, deanbo422@gmail.com
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On 11/7/18, Palmer Dabbelt <palmer@sifive.com> wrote:
> On Mon, 05 Nov 2018 00:52:52 PST (-0800), Arnd Bergmann wrote:
>> On 11/5/18, Christoph Hellwig <hch@infradead.org> wrote:
>>> On Mon, Nov 05, 2018 at 02:58:07PM +0800, Vincent Chen wrote:
>>>> Many thanks for kinds of comments. I quickly synthesize the comments
>>>> and
>>>> list them as below.
>>>> 1. The kernel image shall include all vendor-specific code.
>>>
>>> I fundamentally disagree with this=E2=80=A6 and think it should be the =
contrary.
>>>
>>> 1. The kernel shall support no vendor specific instructions whatsoever,
>>> period.
>>
>> I think what was meant above is
>>
>> 1. If a vendor extension requires kernel support, that support
>> must be able to be built into a kernel image without breaking support
>> for CPUs that do not have that extension, to allow building a single
>> kernel image that works on all CPUs.
>
> Yes.  I don't want anything that won't compile with upstream GCC, but I a=
lso
>
> don't want to have a Kconfig that says "make the kernel only work on
> $VENDOR's  implementation".  I think this can be achieved, at least for t=
he
> cases I've seen so far.

I think over time, the implementations will diverge. Ignoring the question
of vendor extensions for the moment, you will definitely have to deal with
combinations of (future) standard extensions. I can see two ways of
doing that: Either each extension is a separate Kconfig option and you
have to know which one to enable or disable for a particular target,
or you list each platform separately with one Kconfig option, and
have Kconfig/Kbuild work out which features to enable or disable
based on that to get the fastest and most featureful kernel that still
works on all enabled platforms.

       Arnd