Re: [RFC] /dev/ioasid uAPI proposal

[David Gibson <david@gibson.dropbear.id.au>]

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On Thu, Jun 03, 2021 at 09:28:32AM -0300, Jason Gunthorpe wrote:
> On Thu, Jun 03, 2021 at 03:23:17PM +1000, David Gibson wrote:
> > On Wed, Jun 02, 2021 at 01:37:53PM -0300, Jason Gunthorpe wrote:
> > > On Wed, Jun 02, 2021 at 04:57:52PM +1000, David Gibson wrote:
> > > 
> > > > I don't think presence or absence of a group fd makes a lot of
> > > > difference to this design.  Having a group fd just means we attach
> > > > groups to the ioasid instead of individual devices, and we no longer
> > > > need the bookkeeping of "partial" devices.
> > > 
> > > Oh, I think we really don't want to attach the group to an ioasid, or
> > > at least not as a first-class idea.
> > > 
> > > The fundamental problem that got us here is we now live in a world
> > > where there are many ways to attach a device to an IOASID:
> > 
> > I'm not seeing that that's necessarily a problem.
> > 
> > >  - A RID binding
> > >  - A RID,PASID binding
> > >  - A RID,PASID binding for ENQCMD
> > 
> > I have to admit I haven't fully grasped the differences between these
> > modes.  I'm hoping we can consolidate at least some of them into the
> > same sort of binding onto different IOASIDs (which may be linked in
> > parent/child relationships).
> 
> What I would like is that the /dev/iommu side managing the IOASID
> doesn't really care much, but the device driver has to tell
> drivers/iommu what it is going to do when it attaches.

By the device driver, do you mean the userspace or guest device
driver?  Or do you mean the vfio_pci or mdev "shim" device driver"?

> It makes sense, in PCI terms, only the driver knows what TLPs the
> device will generate. The IOMMU needs to know what TLPs it will
> recieve to configure properly.
> 
> PASID or not is major device specific variation, as is the ENQCMD/etc
> 
> Having the device be explicit when it tells the IOMMU what it is going
> to be sending is a major plus to me. I actually don't want to see this
> part of the interface be made less strong.

Ok, if I'm understanding this right a PASID capable IOMMU will be able
to process *both* transactions with just a RID and transactions with a
RID+PASID.

So if we're thinking of this notional 84ish-bit address space, then
that includes "no PASID" as well as all the possible PASID values.
Yes?  Or am I confused?

> 
> > > The selection of which mode to use is based on the specific
> > > driver/device operation. Ie the thing that implements the 'struct
> > > vfio_device' is the thing that has to select the binding mode.
> > 
> > I thought userspace selected the binding mode - although not all modes
> > will be possible for all devices.
> 
> /dev/iommu is concerned with setting up the IOAS and filling the IO
> page tables with information
> 
> The driver behind "struct vfio_device" is responsible to "route" its
> HW into that IOAS.
> 
> They are two halfs of the problem, one is only the io page table, and one
> the is connection of a PCI TLP to a specific io page table.
> 
> Only the driver knows what format of TLPs the device will generate so
> only the driver can specify the "route"

Ok.  I'd really like if we can encode this in a way that doesn't build
PCI-specific structure into the API, though.

>  
> > > eg if two PCI devices are in a group then it is perfectly fine that
> > > one device uses RID binding and the other device uses RID,PASID
> > > binding.
> > 
> > Uhhhh... I don't see how that can be.  They could well be in the same
> > group because their RIDs cannot be distinguished from each other.
> 
> Inability to match the RID is rare, certainly I would expect any IOMMU
> HW that can do PCIEe PASID matching can also do RID matching.

It's not just up to the IOMMU.  The obvious case is a PCIe-to-PCI
bridge.  All transactions show the RID of the bridge, because vanilla
PCI doesn't have them.  Same situation with a buggy multifunction
device which uses function 0's RID for all functions.

It may be rare, but we still have to deal with it one way or another.

I really don't think we want to support multiple binding types for a
single group.

> With
> such HW the above is perfectly fine - the group may not be secure
> between members (eg !ACS), but the TLPs still carry valid RIDs and
> PASID and the IOMMU can still discriminate.

They carry RIDs, whether they're valid depends on how buggy your
hardware is.

> I think you are talking about really old IOMMU's that could only
> isolate based on ingress port or something.. I suppose modern PCIe has
> some cases like this in the NTB stuff too.

Depends what you mean by really old.  They may seem really old to
those working on new fancy IOMMU technology.  But I hit problems in
practice not long ago with awkwardly multi-device groups because it
was on a particular Dell server without ACS implementation.  Likewise
I strongly suspect non-PASID IOMMUs will remain common on low end
hardware (like peoples' laptops) for some time.

> Oh, I hadn't spent time thinking about any of those.. It is messy but
> it can still be forced to work, I guess. A device centric model means
> all the devices using the same routing ID have to be connected to the
> same IOASID by userspace. So some of the connections will be NOPs.

See, that's exactly what I thought the group checks were enforcing.
I'm really hoping we don't need two levels of granularity here: groups
of devices that can't be identified from each other, and then groups
of those that can't be isolated from each other.  That introduces a
huge amount of extra conceptual complexity.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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