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

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On Thu, Sep 21, 2023 at 01:52:24PM -0300, Jason Gunthorpe wrote:
> On Thu, Sep 21, 2023 at 10:43:50AM -0600, Alex Williamson wrote:
> > > With that code in place a legacy driver in the guest has the look and
> > > feel as if having a transitional device with legacy support for both its
> > > control and data path flows.
> > 
> > Why do we need to enable a "legacy" driver in the guest?  The very name
> > suggests there's an alternative driver that perhaps doesn't require
> > this I/O BAR.  Why don't we just require the non-legacy driver in the
> > guest rather than increase our maintenance burden?  Thanks,
> It was my reaction also.
> Apparently there is a big deployed base of people using old guest VMs
> with old drivers and they do not want to update their VMs. It is the
> same basic reason why qemu supports all those weird old machine types
> and HW emulations. The desire is to support these old devices so that
> old VMs can work unchanged.
> Jason

And you are saying all these very old VMs use such a large number of
legacy devices that over-counting of locked memory due to vdpa not
correctly using iommufd is a problem that urgently needs to be solved
otherwise the solution has no value?

Another question I'm interested in is whether there's actually a
performance benefit to using this as compared to just software
vhost. I note there's a VM exit on each IO access, so ... perhaps?
Would be nice to see some numbers.


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