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

From: Peter Maydell
Subject: Re: [RFC 0/3] Introduce a new Qemu machine for RISC-V
Date: Fri, 6 May 2022 11:59:54 +0100

On Fri, 6 May 2022 at 09:18, Daniel P. Berrangé <berrange@redhat.com> wrote:
> On Fri, May 06, 2022 at 06:34:47AM +1000, Alistair Francis wrote:
> > Even if we didn't worry about backwards compatibility the current virt
> > machine would still be what most users want. It's just a small number
> > of users who don't want MMIO devices and instead want to use PCIe for
> > everything. Realistically it's only HPC users who would want this type
> > of machine, at least that's my understanding.
> I'm not so sure about that. Every other architecture has ended up
> standardizing on PCI for general purpose virtual machines. IIRC,
> aarch64 started off with MMIO, but switched to PCI as it matured.
> In terms of having VM mgmt tools "just work" for risc-v, I think
> it will be very compelling for the general 'virt' machine to be
> PCI based, otherwise all the assumptions about PCI in mgmt apps
> are going to break requiring never ending riscv fixes.

Mmm, my experience with aarch64 virt is that PCI is much nicer
as a general preference. aarch64 virt has some MMIO devices
for historical reasons and some because you can't reasonably
do the necessary things with PCI, but I'm actively trying to
push people who submit new MMIO device features for virt to
try to use a PCI-based solution instead if they possibly can.

-- PMM

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