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Re: [Qemu-devel] [PATCH v1 00/21] RISC-V QEMU Port Submission v1

From: Andrea Bolognani
Subject: Re: [Qemu-devel] [PATCH v1 00/21] RISC-V QEMU Port Submission v1
Date: Mon, 08 Jan 2018 16:45:30 +0100

On Wed, 2018-01-03 at 22:06 +0000, Richard W.M. Jones wrote:
> On Thu, Jan 04, 2018 at 10:50:06AM +1300, Michael Clark wrote:
> > > (2) I'm worried that this patch starts off using virtio-mmio instead
> > > of virtio-pci.  virtio-pci is better in every respect than
> > > virtio-mmio, and while it may be a good interim solution I think we
> > > need to have a plan to get rid of it eventually, and should make it
> > > clear that virtio-mmio is not a permanent ABI so we don't get into the
> > > same situation that we did with -M virt on ARM.
> > 
> > I've noted in [PATCH v1 00/00] that we have a goal to add GPEX to the
> > SiFive RISC-V virt board. The idea with the initial version of the virt
> > board is to provide enough infrastructure so that distro builders can use
> > RISC-V QEMU. Prior to the work to implement PLIC, device-tree and VirtIO,
> > there was no network and block device support so it was not possible to use
> > QEMU for distro bring up. Now RISC-V QEMU is usable. Data point: the arm
> > 'virt' board still supports VirtIO MMIO. I don't think this should be a
> > blocking issue.
> I don't think it should be blocking either, but I also think (from
> experience with ARM mach_virt) that we should make it clear that we're
> going to have a flag day down the road where we just remove
> virtio-mmio and everyone will be required to recompile their kernel,
> rather than keep supporting virtio-mmio long term.  However that's
> just my opinion, would like to hear what others say.

Having jumped through many of the same hoops with aarch64/virt as
Rich, although on the libvirt side, I agree with both of you that
virtio-pci should not be a requirement for inclusion: virtio-mmio
is perfectly fine as long as it's very explicitly presented as an
interim solution; the improvements virtio-pci brings to the table
should be plenty to convince people to quickly switch to it, too.

Andrea Bolognani / Red Hat / Virtualization

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