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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Mon, 5 Nov 2018 14:51:33 +0100
Message-ID: <CAK8P3a2MSK-S+AzDGjq6t8WjcKrkj0XmynNL+fMPvL=JgnoEVA@mail.gmail.com>
Subject: Re: [RFC 0/2] RISC-V: A proposal to add vendor-specific code
To:     Christoph Hellwig <hch@infradead.org>
Cc:     Vincent Chen <vincentc@andestech.com>, aou@eecs.berkeley.edu,
        alankao@andestech.com, greentime@andestech.com, palmer@sifive.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 11/5/18, Christoph Hellwig <hch@infradead.org> wrote:
> On Mon, Nov 05, 2018 at 09:52:52AM +0100, Arnd Bergmann wrote:
>> > 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.
>
> No.  This literally means no vendor extensions involving instructions
> or CSRs in the kernel.  They are fine for userspace, or for the M-mode
> code including impementation of the SBI, but not for the kernel.

I was trying to interpret what Vincent wrote, not what you wrote,
you were pretty clear already ;-)

With the stricter policy you suggest, we'd loose the ability to support
some extensions that might be common:

- an extension for user space that adds new registers that must be
  saved and restored on a task switch, e.g. FPU, DSP or NPU
  instructions. ARM supports several incompatible extensions like
  that in one kernel, and this is really ugly, but I suspect RISC-V
  will already need the same thing to support all combinations of
  standard extensions, so from a practical perspective it's not
  much different for custom extension, aside from the question
  how far you want to go to discourage custom extensions by
  requiring users to patch their kernels.

- A crypto instruction for a cipher that is used in the kernel
  for speeding up network or block data encryption.
  This would typically be a standalone loadable module, so
  the impact of allowing custom extensions in addition to
  standard ones is minimal.

       Arnd