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

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Date:   Tue, 30 Jul 2019 16:02:08 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Anup Patel <anup@...infault.org>
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 30/07/19 15:50, Anup Patel wrote:
>> 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.

vhost only needs an eventfd, userspace can poll the eventfd and inject
the irq as usual with KVM_INTERRUPT.  Of course that can be slower, but
you can benchmark it and see if it's indeed a good reason for in-kernel
PLIC.

>> 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.

Sounds good.

Paolo

>>
>>> 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
>