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

Tian, Kevin
2021-08-05T22:44:26Z

> From: Jason Gunthorpe <jgg@nvidia.com>
> Sent: Thursday, August 5, 2021 7:27 PM
>
> On Wed, Aug 04, 2021 at 10:59:21PM +0000, Tian, Kevin wrote:
> > > From: Jason Gunthorpe <jgg@nvidia.com>
> > > Sent: Wednesday, August 4, 2021 10:05 PM
> > >
> > > On Mon, Aug 02, 2021 at 02:49:44AM +0000, Tian, Kevin wrote:
> > >
> > > > Can you elaborate? IMO the user only cares about the label (device
> cookie
> > > > plus optional vPASID) which is generated by itself when doing the
> attaching
> > > > call, and expects this virtual label being used in various spots
> (invalidation,
> > > > page fault, etc.). How the system labels the traffic (the physical RID or
> RID+
> > > > PASID) should be completely invisible to userspace.
> > >
> > > I don't think that is true if the vIOMMU driver is also emulating
> > > PASID. Presumably the same is true for other PASID-like schemes.
> > >
> >
> > I'm getting even more confused with this comment. Isn't it the
> > consensus from day one that physical PASID should not be exposed
> > to userspace as doing so breaks live migration?
>
> Uh, no?
>
> > with PASID emulation vIOMMU only cares about vPASID instead of
> > pPASID, and the uAPI only requires user to register vPASID instead
> > of reporting pPASID back to userspace...
>
> vPASID is only a feature of one device in existance, so we can't make
> vPASID mandatory.
>

sure. my point is just that if vPASID is being emulated there is no need
of exposing pPASID to user space. Can you give a concrete example
where pPASID must be exposed and how the user wants to use this
information?

Thanks
Kevin