linux-kernel - Re: [RFC/PATCH 1/1] virtio: Introduce MMIO ops

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Date:   Thu, 30 Apr 2020 15:34:24 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Srivatsa Vaddagiri <vatsa@...eaurora.org>
Cc:     Jan Kiszka <jan.kiszka@...mens.com>, Will Deacon <will@...nel.org>,
        konrad.wilk@...cle.com, jasowang@...hat.com,
        stefano.stabellini@...inx.com, iommu@...ts.linux-foundation.org,
        virtualization@...ts.linux-foundation.org,
        virtio-dev@...ts.oasis-open.org, tsoni@...eaurora.org,
        pratikp@...eaurora.org, christoffer.dall@....com,
        alex.bennee@...aro.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC/PATCH 1/1] virtio: Introduce MMIO ops

On Thu, Apr 30, 2020 at 07:03:21PM +0530, Srivatsa Vaddagiri wrote:
> * Jan Kiszka <jan.kiszka@...mens.com> [2020-04-30 14:59:50]:
> 
> > >I believe ivshmem2_virtio requires hypervisor to support PCI device emulation
> > >(for life-cycle management of VMs), which our hypervisor may not support.

PCI is mostly just 2 registers. One sets the affected device, one the data to read/write.

> A
> > >simple shared memory and doorbell or message-queue based transport will work for
> > >us.
> > 
> > As written in our private conversation, a mapping of the ivshmem2 device
> > discovery to platform mechanism (device tree etc.) and maybe even the
> > register access for doorbell and life-cycle management to something
> > hypercall-like would be imaginable. What would count more from virtio
> > perspective is a common mapping on a shared memory transport.
> 
> Yes that sounds simpler for us.
> 
> > That said, I also warned about all the features that PCI already defined
> > (such as message-based interrupts) which you may have to add when going a
> > different way for the shared memory device.
> 
> Is it really required to present this shared memory as belonging to a PCI
> device?

But then you will go on and add MSI, and NUMA, and security, and and and ...

> I would expect the device-tree to indicate the presence of this shared
> memory region, which we should be able to present to ivshmem2 as shared memory
> region to use (along with some handles for doorbell or message queue use).
> 
> I understand the usefulness of modeling the shared memory as part of device so
> that hypervisor can send events related to peers going down or coming up. In our
> case, there will be other means to discover those events and avoiding this
> requirement on hypervisor (to emulate PCI) will simplify the solution for us.
> 
> Any idea when we can expect virtio over ivshmem2 to become available?!

Check out the virtio spec. Right at the beginning it states:

	These devices are
	found in virtual environments, yet by design they look like physical devices to the guest within
	the virtual machine - and this document treats them as such. This similarity allows the guest to
	use standard drivers and discovery mechanisms


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