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

[Jason Gunthorpe <jgg@nvidia.com>]

On Thu, Sep 21, 2023 at 04:45:45PM -0400, Michael S. Tsirkin wrote:
> On Thu, Sep 21, 2023 at 04:49:46PM -0300, Jason Gunthorpe wrote:
> > On Thu, Sep 21, 2023 at 03:13:10PM -0400, Michael S. Tsirkin wrote:
> > > On Thu, Sep 21, 2023 at 03:39:26PM -0300, Jason Gunthorpe wrote:
> > > > On Thu, Sep 21, 2023 at 12:53:04PM -0400, Michael S. Tsirkin wrote:
> > > > > > vdpa is not vfio, I don't know how you can suggest vdpa is a
> > > > > > replacement for a vfio driver. They are completely different
> > > > > > things.
> > > > > > Each side has its own strengths, and vfio especially is accelerating
> > > > > > in its capability in way that vpda is not. eg if an iommufd conversion
> > > > > > had been done by now for vdpa I might be more sympathetic.
> > > > > 
> > > > > Yea, I agree iommufd is a big problem with vdpa right now. Cindy was
> > > > > sick and I didn't know and kept assuming she's working on this. I don't
> > > > > think it's a huge amount of work though.  I'll take a look.
> > > > > Is there anything else though? Do tell.
> > > > 
> > > > Confidential compute will never work with VDPA's approach.
> > > 
> > > I don't see how what this patchset is doing is different
> > > wrt to Confidential compute - you trap IO accesses and emulate.
> > > Care to elaborate?
> > 
> > This patch series isn't about confidential compute, you asked about
> > the future. VFIO will support confidential compute in the future, VDPA
> > will not.
> 
> Nonsense it already works.

That isn't what I'm talking about. With a real PCI function and TDISP
we can actually DMA directly from the guest's memory without needing
the ugly bounce buffer hack. Then you can get decent performance.

> But I did not ask about the future since I do not believe it
> can be confidently predicted. I asked what is missing in VDPA
> now for you to add this feature there and not in VFIO.

I don't see that VDPA needs this, VDPA should process the IO BAR on
its own with its own logic, just like everything else it does.

This is specifically about avoiding mediation by relaying directly the
IO BAR operations to the device itself.

That is the entire irony, this whole scheme was designed and
standardized *specifically* to avoid complex mediation and here you
are saying we should just use mediation.

Jason