linux-kernel - Re: [PATCH v18 00/18] KVM RISC-V Support

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Date:   Fri, 21 May 2021 19:21:12 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Palmer Dabbelt <palmerdabbelt@...gle.com>,
        Greg KH <gregkh@...uxfoundation.org>
Cc:     anup@...infault.org, Anup Patel <Anup.Patel@....com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        aou@...s.berkeley.edu, corbet@....net, graf@...zon.com,
        Atish Patra <Atish.Patra@....com>,
        Alistair Francis <Alistair.Francis@....com>,
        Damien Le Moal <Damien.LeMoal@....com>, kvm@...r.kernel.org,
        kvm-riscv@...ts.infradead.org, linux-riscv@...ts.infradead.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-staging@...ts.linux.dev
Subject: Re: [PATCH v18 00/18] KVM RISC-V Support

On 21/05/21 19:13, Palmer Dabbelt wrote:
>> 
> 
> I don't view this code as being in a state where it can be
> maintained, at least to the standards we generally set within the
> kernel.  The ISA extension in question is still subject to change, it
> says so right at the top of the H extension 
> <https://github.com/riscv/riscv-isa-manual/blob/master/src/hypervisor.tex#L4>
> 
>   {\bf Warning! This draft specification may change before being 
>   accepted as standard by the RISC-V Foundation.}

To give a complete picture, the last three relevant changes have been in
August 2019, November 2019 and May 2020.  It seems pretty frozen to me.

In any case, I think it's clear from the experience with Android that
the acceptance policy cannot succeed.  The only thing that such a policy
guarantees, is that vendors will use more out-of-tree code.  Keeping a
fully-developed feature out-of-tree for years is not how Linux is run.

> I'm not sure where exactly the line for real hardware is, but for
> something like this it would at least involve some chip that is
> widely availiable and needs the H extension to be useful

Anup said that "quite a few people have already implemented RISC-V
H-extension in hardware as well and KVM RISC-V works on real HW as 
well".  Those people would benefit from having KVM in the Linus tree.

Paolo