Re: (proposal) RE: [PATCH v7 00/16] vfio: expose virtual Shared Virtual Addressing to VMs

Jason Gunthorpe
2020-10-20T14:02:32Z

On Tue, Oct 20, 2020 at 10:21:41AM +0000, Liu, Yi L wrote:

> > I'm sure there will be some
> > weird overlaps because we can't delete any of the existing VFIO APIs, but
> > that
> > should not be a blocker.
>
> but the weird thing is what we should consider. And it perhaps not just
> overlap, it may be a re-definition of VFIO container. As I mentioned, VFIO
> container is IOMMU context from the day it was defined. It could be the
> blocker. :-(

So maybe you have to broaden the VFIO container to be usable by other
subsystems. The discussion here is about what the uAPI should look
like in a fairly abstract way. When we say 'dev/sva' it just some
placeholder for a shared cdev that provides the necessary
dis-aggregated functionality

It could be an existing cdev with broader functionaltiy, it could
really be /dev/iommu, etc. This is up to the folks building it to
decide.

> I'm not expert on vDPA for now, but I saw you three open source
> veterans have a similar idea for a place to cover IOMMU handling,
> I think it may be a valuable thing to do. I said "may be" as I'm not
> sure about Alex's opinion on such idea. But the sure thing is this
> idea may introduce weird overlap even re-definition of existing
> thing as I replied above. We need to evaluate the impact and mature
> the idea step by step.

This has happened before, uAPIs do get obsoleted and replaced with
more general/better versions. It is often too hard to create a uAPI
that lasts for decades when the HW landscape is constantly changing
and sometime a reset is needed.

The jump to shared PASID based IOMMU feels like one of those moments here.

> > Whoever provides the vIOMMU emulation and relays the page fault to the guest
> > has to translate the RID -
>
> that's the point. But the device info (especially the sub-device info) is
> within the passthru framework (e.g. VFIO). So page fault reporting needs
> to go through passthru framework.
>
> > what does that have to do with VFIO?
> >
> > How will VPDA provide the vIOMMU emulation?
>
> a pardon here. I believe vIOMMU emulation should be based on IOMMU vendor
> specification, right? you may correct me if I'm missing anything.

I'm asking how will VDPA translate the RID when VDPA triggers a page
fault that has to be relayed to the guest. VDPA also has virtual PCI
devices it creates.

We can't rely on VFIO to be the place that the vIOMMU lives because it
excludes/complicates everything that is not VFIO from using that
stuff.

Jason