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

[Robin Murphy]

On 2022-05-31 15:53, Jason Gunthorpe wrote:
> 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..

Eww, no, we can't kill performance by forcing serialisation on the 
entire API just for one silly driver-internal debugfs corner :(

Robin.