Re: [RFC v2] /dev/iommu uAPI proposal

Jason Gunthorpe
2021-07-15T17:18:32Z

On Thu, Jul 15, 2021 at 09:21:41AM -0700, Raj, Ashok wrote:
> On Thu, Jul 15, 2021 at 12:23:25PM -0300, Jason Gunthorpe wrote:
> > On Thu, Jul 15, 2021 at 06:57:57AM -0700, Raj, Ashok wrote:
> > > On Thu, Jul 15, 2021 at 09:48:13AM -0300, Jason Gunthorpe wrote:
> > > > On Thu, Jul 15, 2021 at 06:49:54AM +0000, Tian, Kevin wrote:
> > > >
> > > > > No. You are right on this case. I don't think there is a way to
> > > > > differentiate one mdev from the other if they come from the
> > > > > same parent and attached by the same guest process. In this
> > > > > case the fault could be reported on either mdev (e.g. the first
> > > > > matching one) to get it fixed in the guest.
> > > >
> > > > If the IOMMU can't distinguish the two mdevs they are not isolated
> > > > and would have to share a group. Since group sharing is not supported
> > > > today this seems like a non-issue
> > >
> > > Does this mean we have to prevent 2 mdev's from same pdev being assigned to
> > > the same guest?
> >
> > No, it means that the IOMMU layer has to be able to distinguish them.
>
> Ok, the guest has no control over it, as it see 2 separate pci devices and
> thinks they are all different.
>
> Only time when it can fail is during the bind operation. From guest
> perspective a bind in vIOMMU just turns into a write to local table and a
> invalidate will cause the host to update the real copy from the shadow.
>
> There is no way to fail the bind? and Allocation of the PASID is also a
> separate operation and has no clue how its going to be used in the guest.

You can't attach the same RID to the same PASID twice. The IOMMU code
should prevent this.

As we've talked about several times, it seems to me the vIOMMU
interface is misdesigned for the requirements you have. The hypervisor
should have a role in allocating the PASID since there are invisible
hypervisor restrictions. This is one of them.

> Do we have any isolation requirements here? its the same process. So if the
> page-request it sent to guest and even if you report it for mdev1, after
> the PRQ is resolved by guest, the request from mdev2 from the same guest
> should simply work?

I think we already talked about this and said it should not be done.

Jason