Linux Virtualization - Sep 2023 - [PATCH vfio 11/11] vfio/virtio: Introduce a vfio driver over virtio devices

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.


-- 
MST

> From: Michael S. Tsirkin <mst at redhat.com>
> Sent: Thursday, September 21, 2023 10:31 PM
> 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.
Packet rate and bandwidth are close are only 10% lower than modern device due to
the batching of driver notification.
Bw tested with iperf with one and multiple queues. 
Packet rate tested with testpmd.