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

Jason Gunthorpe
2020-10-20T14:05:18Z

On Tue, Oct 20, 2020 at 02:00:31PM +0000, Liu, Yi L wrote:
> > From: Jason Gunthorpe <jgg@nvidia.com>
> > Sent: Tuesday, October 20, 2020 9:55 PM
> >
> > On Tue, Oct 20, 2020 at 09:40:14AM +0000, Liu, Yi L wrote:
> >
> > > > See previous discussion with Kevin. If I understand correctly, you expect a
> > shared
> > > > L2 table if vDPA and VFIO device are using the same PASID.
> > >
> > > L2 table sharing is not mandatory. The mapping is the same, but no need to
> > > assume L2 tables are shared. Especially for VFIO/vDPA devices. Even within
> > > a passthru framework, like VFIO, if the attributes of backend IOMMU are not
> > > the same, the L2 page table are not shared, but the mapping is the same.
> >
> > I think not being able to share the PASID shows exactly why this VFIO
> > centric approach is bad.
>
> no, I didn't say PASID is not sharable. My point is sharing L2 page table is
> not mandatory.

IMHO a PASID should be 1:1 with a page table, what does it even mean
to share a PASID but have different page tables?

Jason