Re: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and allocation APIs

[Jason Gunthorpe]

On Fri, Apr 02, 2021 at 08:22:28AM +0000, Tian, Kevin wrote:
> > From: Jason Gunthorpe <jgg@xxxxxxxxxx>
> > Sent: Tuesday, March 30, 2021 9:29 PM
> > 
> > >
> > > First, userspace may use ioasid in a non-SVA scenario where ioasid is
> > > bound to specific security context (e.g. a control vq in vDPA) instead of
> > > tying to mm. In this case there is no pgtable binding initiated from user
> > > space. Instead, ioasid is allocated from /dev/ioasid and then programmed
> > > to the intended security context through specific passthrough framework
> > > which manages that context.
> > 
> > This sounds like the exact opposite of what I'd like to see.
> > 
> > I do not want to see every subsystem gaining APIs to program a
> > PASID. All of that should be consolidated in *one place*.
> > 
> > I do not want to see VDPA and VFIO have two nearly identical sets of
> > APIs to control the PASID.
> > 
> > Drivers consuming a PASID, like VDPA, should consume the PASID and do
> > nothing more than authorize the HW to use it.
> > 
> > quemu should have general code under the viommu driver that drives
> > /dev/ioasid to create PASID's and manage the IO mapping according to
> > the guest's needs.
> > 
> > Drivers like VDPA and VFIO should simply accept that PASID and
> > configure/authorize their HW to do DMA's with its tag.
> > 
> 
> I agree with you on consolidating things in one place (especially for the
> general SVA support). But here I was referring to an usage without 
> pgtable binding (Possibly Jason. W can say more here), where the 
> userspace just wants to allocate PASIDs, program/accept PASIDs to 
> various workqueues (device specific), and then use MAP/UNMAP 
> interface to manage address spaces associated with each PASID. 
> I just wanted to point out that the latter two steps are through 
> VFIO/VDPA specific interfaces. 

No, don't do that.

VFIO and VDPA has no buisness having map/unmap interfaces once we have
/dev/ioasid. That all belongs in the iosaid side.

I know they have those interfaces today, but that doesn't mean we have
to keep using them for PASID use cases, they should be replaced with a
'do dma from this pasid on /dev/ioasid' interface certainly not a
'here is a pasid from /dev/ioasid, go ahead and configure it youself'
interface

This is because PASID is *complicated* in the general case! For
instance all the two level stuff you are talking about must not leak
into every user!

Jason