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

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On Fri, Sep 22, 2023 at 6:55 AM Jason Gunthorpe <jgg@xxxxxxxxxx> wrote:
>
> 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.

What blocks vDPA from supporting that?

> >
> > 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.

This series requires the trapping in the legacy I/O BAR in VFIO. Why
can TDISP work when trapping in VFIO but not vDPA? If neither, how can
TDISP affect here?

>
> > 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.

So we had:

1) a new virtio specific driver for VFIO
2) the existing vp_vdpa driver

How much differences between them in the context of the mediation or
relaying? Or is it hard to introduce admin commands in the vDPA bus?

> 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.

No, using "simple VFIO passthrough" is just fine.

Thanks

>
> Jason
>





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