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

From: Atish Kumar Patra <atishp@rivosinc.com>
To: Peter Maydell <peter.maydell@linaro.org>
Cc: "Daniel P. Berrangé" <berrange@redhat.com>,
	"Alistair Francis" <alistair23@gmail.com>,
	"Atish Patra" <atishp@atishpatra.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: Fri, 6 May 2022 13:30:01 -0700	[thread overview]
Message-ID: <CAHBxVyH=7hX5mjWcUBODk_tNHoczefdkfS8dFcf6is=vpz=SZw@mail.gmail.com> (raw)
In-Reply-To: <CAFEAcA_MncAJWUqG3yRVGMT0mXKvnBUqJLdoHwMdd=OW8fnodQ@mail.gmail.com>

On Fri, May 6, 2022 at 4:00 AM Peter Maydell <peter.maydell@linaro.org> wrote:
>
> 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.
>

Yeah. That was one of the primary goals of this series. If we have an
alternate PCI only machine,
folks will be more motivated to add only PCI based solutions in the
future. In that case, there will be minimal
or no change required to the machine code itself.

Just for clarification: We can achieve the same with the current virt
machine. But it is already bloated with a bunch of MMIO devices
and will probably continue to do so because of its flexibility. In
addition to that, any real PCI based platform emulation can not reuse
the virt machine code which will result in more vendor specific
implementations in the future..

> thanks
> -- PMM