linux-kernel - Re: [RFC PATCH 00/16] KVM RISC-V Support

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Date:   Tue, 30 Jul 2019 19:20:15 +0530
From:   Anup Patel <anup@...infault.org>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Anup Patel <Anup.Patel@....com>,
        Palmer Dabbelt <palmer@...ive.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Radim K <rkrcmar@...hat.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Atish Patra <Atish.Patra@....com>,
        Alistair Francis <Alistair.Francis@....com>,
        Damien Le Moal <Damien.LeMoal@....com>,
        Christoph Hellwig <hch@...radead.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 00/16] KVM RISC-V Support

On Tue, Jul 30, 2019 at 5:03 PM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 30/07/19 07:26, Anup Patel wrote:
> > Here's a brief TODO list which we want to immediately work upon after this
> > series:
> > 1. Handle trap from unpriv access in SBI v0.1 emulation
> > 2. In-kernel PLIC emulation
> > 3. SBI v0.2 emulation in-kernel
> > 4. SBI v0.2 hart hotplug emulation in-kernel
> > 5. ..... and so on .....
> >
> > We will include above TODO list in v2 series cover letter as well.
>
> I guess I gave you a bunch of extra items in today's more thorough
> review. :)

Thanks, your review comments are very useful. We will address all
of them.

>
> BTW, since IPIs are handled in the SBI I wouldn't bother with in-kernel
> PLIC emulation unless you can demonstrate performance improvements (for
> example due to irqfd).  In fact, it may be more interesting to add

I thought VHOST requires irqfd and we would certainly endup providing
in-kernel PLIC emulation to support VHOST.

> plumbing for userspace handling of selected SBI calls (in addition to
> get/putchar, sbi_system_reset and sbi_hart_down look like good
> candidates in SBI v0.2).

The get/putchar SBI v0.1 calls won't be encouraged going forward because
we already have earlycon implmentation in-place and Guest kernel can directly
write to UART registers for earlyprints.

If we still wanted to implement get/putchar calls then we would need a RISC-V
specific exit reason in KVM. We have tried to avoid RISC-V specific IOCTLs
or exit reason in this series.

>
> > We were thinking to keep KVM RISC-V disabled by default (i.e. keep it
> > experimental) until we have validated it on some FPGA or real HW. For now,
> > users can explicitly enable it and play-around on QEMU emulation. I hope
> > this is fine with most people ?
>
> That's certainly okay with me.
>

Thanks,
Anup