Re: [RFC 0/3] Introduce a new Qemu machine for RISC-V

From: Alistair Francis <alistair23@gmail.com>
To: Andrea Bolognani <abologna@redhat.com>
Cc: "Atish Patra" <atishp@atishpatra.org>,
	"Daniel P. Berrangé" <berrange@redhat.com>,
	"Atish Kumar Patra" <atishp@rivosinc.com>,
	"Peter Maydell" <peter.maydell@linaro.org>,
	"open list:RISC-V" <qemu-riscv@nongnu.org>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	"Bin Meng" <bin.meng@windriver.com>,
	"qemu-devel@nongnu.org Developers" <qemu-devel@nongnu.org>,
	"Alistair Francis" <alistair.francis@wdc.com>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	"Palmer Dabbelt" <palmer@dabbelt.com>
Subject: Re: [RFC 0/3] Introduce a new Qemu machine for RISC-V
Date: Wed, 1 Jun 2022 12:01:56 +1000	[thread overview]
Message-ID: <CAKmqyKMEFryRVCEhMniYA0RH46CMmpowK8s_QJ72YcP2J+mJ7w@mail.gmail.com> (raw)
In-Reply-To: <CABJz62NR=6+k0AxZ4V35hXH=bUamZR4=f4wgDqcG8zm0e+KmHg@mail.gmail.com>

On Wed, May 25, 2022 at 1:56 AM Andrea Bolognani <abologna@redhat.com> wrote:
>
> On Mon, May 23, 2022 at 08:16:40PM -0700, Atish Patra wrote:
> > On Sun, May 22, 2022 at 10:59 PM Alistair Francis <alistair23@gmail.com> wrote:
> > > On Wed, May 18, 2022 at 4:38 PM Atish Patra <atishp@atishpatra.org> wrote:
> > > > 1. virt machine is not well documented and already bloated. There is
> > > > no specification for virt machine as such.
> > > > Putting restrictions after a certain release will lead to confusion.
> > > > 2. Do we support existing MMIO devices after that specific version or not ?
> > >
> > > Yeah, so I guess this doesn't achieve the same outcome you want. I
> > > would say we would still include some MMIO devices, like UART for
> > > example.
> >
> > Why ? We can just rely on the pcie based uart (virtio-serial-pci or
> > serial-pci) should be enough.
> > The only MMIO devices that should be allowed are the ones that can't
> > be behind pcie.
>
> IIRC virtio-serial is initialized too late to catch messages produced
> very early by the firmware (and possibly the kernel), which means
> it's okay for regular usage but not when trying to debug an entire
> class of boot issues.

Agreed. OpenSBI doesn't even support PCIe so we need an MMIO UART for
OpenSBI to be able to print messages

Alistair

>
> Either way, it looks like you wouldn't be able to completely get rid
> of MMIO even if you introduced a new virt-pcie machine type. That's
> the same for the aarch64 virt machine. I agree with Dan that we
> should follow the example set by that architecture - it has worked
> out pretty well.
>
> If there is a desire to reduce the complexity of the "standard"
> machine type, can we just take the current virt machine type and
> rename it to something else? And have your simpler code take over the
> virt name? Sure, it will cause some pain in the short term, but the
> RISC-V ecosystem is still young enough for it to not be a deal
> breaker.
>
> --
> Andrea Bolognani / Red Hat / Virtualization
>