Re: Plan for /dev/ioasid RFC v2

[Lu Baolu]

On 6/9/21 8:39 PM, Jason Gunthorpe wrote:
> On Wed, Jun 09, 2021 at 02:24:03PM +0200, Joerg Roedel wrote:
> > On Mon, Jun 07, 2021 at 02:58:18AM +0000, Tian, Kevin wrote:
> > > -   Device-centric (Jason) vs. group-centric (David) uAPI. David is not fully
> > >      convinced yet. Based on discussion v2 will continue to have ioasid uAPI
> > >      being device-centric (but it's fine for vfio to be group-centric). A new
> > >      section will be added to elaborate this part;
> > I would vote for group-centric here. Or do the reasons for which VFIO is
> > group-centric not apply to IOASID? If so, why?
> VFIO being group centric has made it very ugly/difficult to inject
> device driver specific knowledge into the scheme.
>
> The device driver is the only thing that knows to ask:
>   - I need a SW table for this ioasid because I am like a mdev
>   - I will issue TLPs with PASID
>   - I need a IOASID linked to a PASID
>   - I am a devices that uses ENQCMD and vPASID
>   - etc in future
>
> The current approach has the group try to guess the device driver
> intention in the vfio type 1 code.
>
> I want to see this be clean and have the device driver directly tell
> the iommu layer what kind of DMA it plans to do, and thus how it needs
> the IOMMU and IOASID configured.
>
> This is the source of the ugly symbol_get and the very, very hacky 'if
> you are a mdev*and*  a iommu then you must want a single PASID' stuff
> in type1.
>
> The group is causing all this mess because the group knows nothing
> about what the device drivers contained in the group actually want.
>
> Further being group centric eliminates the possibility of working in
> cases like !ACS. How do I use PASID functionality of a device behind a
> !ACS switch if the uAPI forces all IOASID's to be linked to a group,
> not a device?
>
> Device centric with an report that "all devices in the group must use
> the same IOASID" covers all the new functionality, keep the old, and
> has a better chance to keep going as a uAPI into the future.

The iommu_group can guarantee the isolation among different physical
devices (represented by RIDs). But when it comes to sub-devices (ex. 
mdev or vDPA devices represented by RID + SSID), we have to rely on the
device driver for isolation. The devices which are able to generate sub-
devices should either use their own on-device mechanisms or use the
platform features like Intel Scalable IOV to isolate the sub-devices.

Under above conditions, different sub-device from a same RID device
could be able to use different IOASID. This seems to means that we can't
support mixed mode where, for example, two RIDs share an iommu_group and
one (or both) of them have sub-devices.

AIUI, when we attach a "RID + SSID" to an IOASID, we should require that
the RID doesn't share the iommu_group with any other RID.

Best regards,
baolu