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

Liu, Yi L
2020-10-20T14:09:56Z

> From: Jason Gunthorpe <jgg@nvidia.com>
> Sent: Tuesday, October 20, 2020 10:05 PM
> To: Liu, Yi L <yi.l.liu@intel.com>
>
> 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?

PASID is actually 1:1 with an address space. Not really needs to be 1:1 with
page table. :-)

Regards,
Yi Liu

> Jason