On Mon, Jul 06, 2015 at 05:29:39PM +0200, Daniel Vetter wrote: > On Mon, Jul 06, 2015 at 03:57:44PM +0100, Chris Wilson wrote: > > On Mon, Jul 06, 2015 at 05:50:37PM +0300, Imre Deak wrote: > > > We have 3 types of DMA mappings for GEM objects: > > > 1. physically contiguous for stolen and for objects needing contiguous > > > memory > > > 2. DMA-buf mappings imported via a DMA-buf attach operation > > > 3. SG DMA mappings for shmem backed and userptr objects > > > > > > For 1. and 2. the lifetime of the DMA mapping matches the lifetime of the > > > corresponding backing pages and so in practice we create/release the > > > mapping in the object's get_pages/put_pages callback. > > > > > > For 3. the lifetime of the mapping matches that of any existing GPU binding > > > of the object, so we'll create the mapping when the object is bound to > > > the first vma and release the mapping when the object is unbound from its > > > last vma. > > > > > > Since the object can be bound to multiple vmas, we can end up creating a > > > new DMA mapping in the 3. case even if the object already had one. This > > > is not allowed by the DMA API and can lead to leaked mapping data and > > > IOMMU memory space starvation in certain cases. For example HW IOMMU > > > drivers (intel_iommu) allocate a new range from their memory space > > > whenever a mapping is created, silently overriding a pre-existing > > > mapping. > > How does this happen? Essentially list_empty(obj->vmas) == > !dma_mapping_exists should hold for objects of the 3rd type. I don't > understand how this is broken in the current code. There was definitely > versions of the ppgtt code where this wasn't working properly, but I > thought we've fixed that up again. Every g/ppgtt binding remapped the obj->pages through the iommu. Even with the DMAR disabled, we still pay the cpu cost of sw iommu (which is itself an annoying kernel bug that you can't disable). > > > Fix this by adding new callbacks to create/release the DMA mapping. This > > > way we can use the has_dma_mapping flag for objects of the 3. case also > > > (so far the flag was only used for the 1. and 2. case) and skip creating > > > a new mapping if one exists already. > > > > > > Note that I also thought about simply creating/releasing the mapping > > > when get_pages/put_pages is called. However since creating a DMA mapping > > > may have associated resources (at least in case of HW IOMMU) it does > > > make sense to release these resources as early as possible. We can > > > release the DMA mapping as soon as the object is unbound from the last > > > vma, before we drop the backing pages, hence it's worth keeping the two > > > operations separate. > > > > > > I noticed this issue by enabling DMA debugging, which got disabled after > > > a while due to its internal mapping tables getting full. It also reported > > > errors in connection to random other drivers that did a DMA mapping for > > > an address that was previously mapped by i915 but was never released. > > > Besides these diagnostic messages and the memory space starvation > > > problem for IOMMUs, I'm not aware of this causing a real issue. > > > > Nope, it is much much simpler. Since we only do the dma prepare/finish > > from inside get_pages/put_pages, we can put the calls there. The only > > caveat there is userptr worker, but that can be easily fixed up. > > I do kinda like the distinction between just grabbing the backing storage > and making it accessible to the hw. Small one, but I think it does help if > we keep these two maps separate. Now the function names otoh are > super-confusing, that I agree with. But that is the raison-d'etre of get_pages(). We call it preciselly when we want the backing storage available to the hw. We relaxed that for set-domain to avoid one type of bug, and stolen/dma-buf have their own notion of dma mapping. userptr is the odd one out due to its worker asynchronously grabbing the pages. -Chris -- Chris Wilson, Intel Open Source Technology Centre _______________________________________________ Intel-gfx mailing list Intel-gfx@xxxxxxxxxxxxxxxxxxxxx http://lists.freedesktop.org/mailman/listinfo/intel-gfx
