RE: [RFC] /dev/ioasid uAPI proposal

[Tian, Kevin]

> From: Jean-Philippe Brucker <jean-philippe@xxxxxxxxxx>
> Sent: Friday, June 4, 2021 4:18 PM
> 
> On Wed, Jun 02, 2021 at 01:25:00AM +0000, Tian, Kevin wrote:
> > > > This implies that VFIO_BOUND_IOASID will be extended to allow user
> > > > specify a device label. This label will be recorded in /dev/iommu to
> > > > serve per-device invalidation request from and report per-device
> > > > fault data to the user.
> > >
> > > I wonder which of the user providing a 64 bit cookie or the kernel
> > > returning a small IDA is the best choice here? Both have merits
> > > depending on what qemu needs..
> >
> > Yes, either way can work. I don't have a strong preference. Jean?
> 
> I don't see an issue with either solution, maybe it will show up while
> prototyping. First one uses IDs that do mean something for someone, and
> userspace may inject faults slightly faster since it doesn't need an
> ID->vRID lookup, so that's my preference.

ok, will go for the first option in v2.

> 
> > > > In addition, vPASID (if provided by user) will
> > > > be also recorded in /dev/iommu so vPASID<->pPASID conversion
> > > > is conducted properly. e.g. invalidation request from user carries
> > > > a vPASID which must be converted into pPASID before calling iommu
> > > > driver. Vice versa for raw fault data which carries pPASID while the
> > > > user expects a vPASID.
> > >
> > > I don't think the PASID should be returned at all. It should return
> > > the IOASID number in the FD and/or a u64 cookie associated with that
> > > IOASID. Userspace should figure out what the IOASID & device
> > > combination means.
> >
> > This is true for Intel. But what about ARM which has only one IOASID
> > (pasid table) per device to represent all guest I/O page tables?
> 
> In that case vPASID = pPASID though. The vPASID allocated by the guest is
> the same from the vIOMMU inval to the pIOMMU inval. I don't think host
> kernel or userspace need to alter it.
> 

yes. So responding to Jason's earlier comment we do need return
PASID (although no conversion is required) to userspace in this
case. 😊

Thanks
Kevin