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

[Jacob Pan]

Hi Jason,

On Wed, 31 Mar 2021 09:28:05 -0300, Jason Gunthorpe <jgg@xxxxxxxxxx> wrote:

> On Tue, Mar 30, 2021 at 05:10:41PM -0700, Jacob Pan wrote:
>  [...]  
>  [...]  
>  [...]  
> > This requires the mdev driver to obtain a list of allowed
> > PASIDs(possibly during PASID bind time) prior to do enforcement. IMHO,
> > the PASID enforcement points are:
> > 1. During WQ configuration (e.g.program MSI)
> > 2. During work submission
> > 
> > For VT-d shared workqueue, there is no way to enforce #2 in mdev driver
> > in that the PASID is obtained from PASID MSR from the CPU and submitted
> > w/o driver involvement.  
> 
> I assume that the PASID MSR is privileged and only qemu can program
> it? Otherwise this seems like a security problem.
> 
yes.

> If qemu controls it then the idxd userspace driver in qemu must ensure
> it is only ever programmed to an authorized PASID.
> 
it is ensured for #1.

> > The enforcement for #2 is in the KVM PASID translation table, which
> > is per VM.  
> 
> I don't understand why KVM gets involved in PASID??
> 
Here is an excerpt from the SIOV spec.
https://software.intel.com/content/www/us/en/develop/download/intel-scalable-io-virtualization-technical-specification.html

"3.3 PASID translation
To support PASID isolation for Shared Work Queues used by VMs, the CPU must
provide a way for the PASID to be communicated to the device in the DMWr
transaction. On Intel CPUs, the CPU provides a PASID translation table in
the vCPUs virtual machine control structures. During ENQCMD/ENQCMDS
instruction execution in a VM, the PASID translation table is used by the
CPU to replace the guest PASID in the work descriptor with a host PASID
before the descriptor is sent to the device.3.3 PASID translation"

> Doesn't work submission go either to the mdev driver or through the
> secure PASID of #1?
> 
No, once a PASID is bound with IOMMU, KVM, and the mdev, work submission is
all done in HW.
But I don't think this will change for either uAPI design.

> > For our current VFIO mdev model, bind guest page table does not involve
> > mdev driver. So this is a gap we must fill, i.e. include a callback from
> > mdev driver?  
> 
> No not a callback, tell the mdev driver with a VFIO IOCTL that it is
> authorized to use a specific PASID because the vIOMMU was told to
> allow it by the guest kernel. Simple and straightforward.
> 
Make sense.

> > > ioasid_set doesn't seem to help at all, certainly not as a concept
> > > tied to /dev/ioasid.
> > >   
> > Yes, we can take the security role off ioasid_set once we have per mdev
> > list. However, ioasid_set being a per VM/mm entity also bridge
> > communications among kernel subsystems that don't have direct call path.
> > e.g. KVM, VDCM and IOMMU.  
> 
> Everything should revolve around the /dev/ioasid FD. qemu should pass
> it to all places that need to know about PASID's in the VM.
> 
I guess we need to extend KVM interface to support PASIDs. Our original
intention was to avoid introducing new interfaces.

> We should try to avoid hidden behind the scenes kernel
> interconnections between subsystems.
> 
Can we? in case of exception. Since all these IOCTLs are coming from the
unreliable user space, we must deal all exceptions.

For example, when user closes /dev/ioasid FD before (or w/o) unbind IOCTL
for VFIO, KVM, kernel must do cleanup and coordinate among subsystems.
In this patchset, we have a per mm(ioasid_set) notifier to inform mdev, KVM
to clean up and drop its refcount. Do you have any suggestion on this?

> 
> > > So when you 'allow' a mdev to access a PASID you want to say:
> > >  Allow Guest PASID A, map it to host PASID B on this /dev/ioasid FD
> > >   
> 
> > Host and guest PASID value, as well as device info are available through
> > iommu_uapi_sva_bind_gpasid(), we just need to feed that info to mdev
> > driver.  
> 
> You need that IOCTL to exist on the *mdev driver*. It is a VFIO ioctl,
> not a iommu or ioasid or sva IOCTL.
>
OK. A separate IOCTL and separate step.

> > > That seems like a good helper library to provide for drivers to use,
> > > but it should be a construct entirely contained in the driver.  
> > why? would it be cleaner if it is in the common code?  
> 
> No, it is the "mid layer" problematic design.
> 
> Having the iommu layer store driver-specific data on behalf of a
> driver will just make a mess. Use the natural layering we have and
> store driver specific data in the driver structs.
> 
> Add a library to help build the datastructure if it necessary.
> 
Let me try to paraphrase, you are suggesting common helper code and data
format but still driver specific storage of the mapping, correct?

Will try this out, seems cleaner.

> Jason


Thanks,

Jacob