Re: [PATCH vfio 11/11] vfio/virtio: Introduce a vfio driver over virtio devices

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Sep 25, 2023 at 03:53:18PM -0300, Jason Gunthorpe wrote:
> On Mon, Sep 25, 2023 at 02:16:30PM -0400, Michael S. Tsirkin wrote:
> 
> > I do want to understand if there's a use-case that vdpa does not address
> > simply because it might be worth while to extend it to do so, and a
> > bunch of people working on it are at Red Hat and I might have some input
> > into how that labor is allocated. But if the use-case is simply "has to
> > be vfio and not vdpa" then I guess not.
> 
> If you strip away all the philisophical arguing VDPA has no way to
> isolate the control and data virtqs to different IOMMU configurations
> with this single PCI function.

Aha, so address space/PASID support then?

> The existing HW VDPA drivers provided device specific ways to handle
> this.
> 
> Without DMA isolation you can't assign the high speed data virtq's to
> the VM without mediating them as well.
> 
> > It could be that we are using mediation differently - in my world it's
> > when there's some host software on the path between guest and hardware,
> > and this qualifies.  
> 
> That is pretty general. As I said to Jason, if you want to use it that
> way then you need to make up a new word to describe what VDPA does as
> there is a clear difference in scope between this VFIO patch (relay IO
> commands to the device) and VDPA (intercept all the control plane,
> control virtq and bring it to a RedHat/qemu standard common behavior)

IIUC VDPA itself does not really bring it to either RedHat or qemu
standard, it just allows userspace to control behaviour - if userspace
is qemu then it's qemu deciding how it behaves. Which I guess this
doesn't. Right?  RedHat's not in the picture at all I think.

> > There is also a question of capability. Specifically iommufd support
> > is lacking in vdpa (though there are finally some RFC patches to
> > address that). All this is fine, could be enough to motivate
> > a work like this one.
> 
> I've answered many times, you just don't semm to like the answers or
> dismiss them as not relevant to you.
> 
> Jason


Not really I think I lack some of the picture so I don't fully
understand. Or maybe I missed something else.

-- 
MST

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