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

Raj, Ashok
2021-07-15T17:49:12Z

On Thu, Jul 15, 2021 at 02:18:26PM -0300, Jason Gunthorpe wrote:
> 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.

Allocating a PASID is a separate step from binding, isn't it? In vt-d we
have a virtual command interface that can fail an allocation of PASID. But
which device its bound to is a dynamic thing that only gets at bind_mm()
right?

>
> > 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.

I get the should not be done, I'm wondering where should that be
implemented?