On Thu, Aug 15, 2019 at 07:21:47PM +0200, Daniel Vetter wrote: > On Thu, Aug 15, 2019 at 7:16 PM Jason Gunthorpe <jgg@xxxxxxxx> wrote: > > > > On Thu, Aug 15, 2019 at 12:32:38PM -0400, Jerome Glisse wrote: > > > On Thu, Aug 15, 2019 at 12:10:28PM -0300, Jason Gunthorpe wrote: > > > > On Thu, Aug 15, 2019 at 04:43:38PM +0200, Daniel Vetter wrote: > > > > > > > > > You have to wait for the gpu to finnish current processing in > > > > > invalidate_range_start. Otherwise there's no point to any of this > > > > > really. So the wait_event/dma_fence_wait are unavoidable really. > > > > > > > > I don't envy your task :| > > > > > > > > But, what you describe sure sounds like a 'registration cache' model, > > > > not the 'shadow pte' model of coherency. > > > > > > > > The key difference is that a regirstationcache is allowed to become > > > > incoherent with the VMA's because it holds page pins. It is a > > > > programming bug in userspace to change VA mappings via mmap/munmap/etc > > > > while the device is working on that VA, but it does not harm system > > > > integrity because of the page pin. > > > > > > > > The cache ensures that each initiated operation sees a DMA setup that > > > > matches the current VA map when the operation is initiated and allows > > > > expensive device DMA setups to be re-used. > > > > > > > > A 'shadow pte' model (ie hmm) *really* needs device support to > > > > directly block DMA access - ie trigger 'device page fault'. ie the > > > > invalidate_start should inform the device to enter a fault mode and > > > > that is it. If the device can't do that, then the driver probably > > > > shouldn't persue this level of coherency. The driver would quickly get > > > > into the messy locking problems like dma_fence_wait from a notifier. > > > > > > I think here we do not agree on the hardware requirement. For GPU > > > we will always need to be able to wait for some GPU fence from inside > > > the notifier callback, there is just no way around that for many of > > > the GPUs today (i do not see any indication of that changing). > > > > I didn't say you couldn't wait, I was trying to say that the wait > > should only be contigent on the HW itself. Ie you can wait on a GPU > > page table lock, and you can wait on a GPU page table flush completion > > via IRQ. > > > > What is troubling is to wait till some other thread gets a GPU command > > completion and decr's a kref on the DMA buffer - which kinda looks > > like what this dma_fence() stuff is all about. A driver like that > > would have to be super careful to ensure consistent forward progress > > toward dma ref == 0 when the system is under reclaim. > > > > ie by running it's entire IRQ flow under fs_reclaim locking. > > This is correct. At least for i915 it's already a required due to our > shrinker also having to do the same. I think amdgpu isn't bothering > with that since they have vram for most of the stuff, and just limit > system memory usage to half of all and forgo the shrinker. Probably > not the nicest approach. Anyway, both do the same mmu_notifier dance, > just want to explain that we've been living with this for longer > already. > > So yeah writing a gpu driver is not easy. > > > > associated with the mm_struct. In all GPU driver so far it is a short > > > lived lock and nothing blocking is done while holding it (it is just > > > about updating page table directory really wether it is filling it or > > > clearing it). > > > > The main blocking I expect in a shadow PTE flow is waiting for the HW > > to complete invalidations of its PTE cache. > > > > > > It is important to identify what model you are going for as defining a > > > > 'registration cache' coherence expectation allows the driver to skip > > > > blocking in invalidate_range_start. All it does is invalidate the > > > > cache so that future operations pick up the new VA mapping. > > > > > > > > Intel's HFI RDMA driver uses this model extensively, and I think it is > > > > well proven, within some limitations of course. > > > > > > > > At least, 'registration cache' is the only use model I know of where > > > > it is acceptable to skip invalidate_range_end. > > > > > > Here GPU are not in the registration cache model, i know it might looks > > > like it because of GUP but GUP was use just because hmm did not exist > > > at the time. > > > > It is not because of GUP, it is because of the lack of > > invalidate_range_end. A driver cannot correctly implement the SPTE > > model without invalidate_range_end, even if it holds the page pins via > > GUP. > > > > So, I've been assuming the few drivers without invalidate_range_end > > are trying to do registration caching, rather than assuming they are > > broken. > > I915 might just be broken. amdgpu does the full thing, using > hmm_mirror. But still with dma_fence_wait. Yeah i915 is broken but it never hurted anyone ;) I posted patch a long time ago to convert it to hmm but i delayed that to until i can get through making something of GUPfast that can also be use for HMM/ODP user. Cheers, Jérôme _______________________________________________ Intel-gfx mailing list Intel-gfx@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/intel-gfx
