Re: [PATCH 01/12] iommu/vt-d: Use iommu_get_domain_for_dev() in debugfs

Jason Gunthorpe via iommu
2022-05-31T14:53:09Z

On Tue, May 31, 2022 at 10:11:18PM +0800, Baolu Lu wrote:
> On 2022/5/31 21:10, Jason Gunthorpe wrote:
> > On Tue, May 31, 2022 at 11:02:06AM +0800, Baolu Lu wrote:
> >
> > > For case 2, it is a bit weird. I tried to add a rwsem lock to make the
> > > iommu_unmap() and dumping tables in debugfs exclusive. This does not
> > > work because debugfs may depend on the DMA of the devices to work. It
> > > seems that what we can do is to allow this race, but when we traverse
> > > the page table in debugfs, we will check the validity of the physical
> > > address retrieved from the page table entry. Then, the worst case is to
> > > print some useless information.
> >
> > Sounds horrible, don't you have locking around the IOPTEs of some
> > kind? How does updating them work reliably?
>
> There's no locking around updating the IOPTEs. The basic assumption is
> that at any time, there's only a single thread manipulating the mappings
> of the range specified in iommu_map/unmap() APIs. Therefore, the race
> only exists when multiple ranges share some high-level IOPTEs. The IOMMU
> driver updates those IOPTEs using the compare-and-exchange atomic
> operation.

Oh? Did I miss where that was documented as part of the iommu API?

Daniel posted patches for VFIO to multi-thread iommu_domin mapping.

iommufd goes out of its way to avoid this kind of serialization so
that userspace can parallel map IOVA.

I think if this is the requirement then the iommu API needs to
provide a lock around the domain for the driver..

Jason