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

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Date:   Fri, 21 May 2021 19:47:51 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Palmer Dabbelt <palmerdabbelt@...gle.com>, 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 Fri, May 21, 2021 at 07:21:12PM +0200, Paolo Bonzini wrote:
> 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.

Great, but is this really true?  If so, what hardware has this?  I have
a new RISC-V device right here next to me, what would I need to do to
see if this is supported in it or not?

If this isn't in any hardware that anyone outside of
internal-to-company-prototypes, then let's wait until it really is in a
device that people can test this code on.

What's the rush to get this merged now if no one can use it?

thanks,

greg k-h