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

[Jason Wang]

在 2021/4/6 上午7:42, Jason Gunthorpe 写道:
> 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


So it looks like the PASID was bound to SVA in this design. I think it's 
not necessairly the case:

1) PASID can be implemented without SVA, in this case a map/unmap 
interface is still required
2) For the case that hypervisor want to do some mediation in the middle 
for a virtqueue. e.g in the case of control vq that is implemented in 
the VF/ADI/SF itself, the hardware virtqueue needs to be controlled by 
Qemu, Though binding qemu's page table to cvq can work but it looks like 
a overkill, a small dedicated buffers that is mapped for this PASID 
seems more suitalbe.


> 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


So do you mean the device should not expose the PASID confiugration API 
to guest? I think it could happen if we assign the whole device and let 
guest to configure it for nested VMs.

Thanks


>