On Wed, Dec 18, 2019 at 02:41:47PM -0800, Niranjana Vishwanathapura wrote:
> > > +static u32 i915_svm_build_sg(struct i915_address_space *vm,
> > > + struct hmm_range *range,
> > > + struct sg_table *st)
> > > +{
> > > + struct scatterlist *sg;
> > > + u32 sg_page_sizes = 0;
> > > + u64 i, npages;
> > > +
> > > + sg = NULL;
> > > + st->nents = 0;
> > > + npages = (range->end - range->start) / PAGE_SIZE;
> > > +
> > > + /*
> > > + * No need to dma map the host pages and later unmap it, as
> > > + * GPU is not allowed to access it with SVM.
> > > + * XXX: Need to dma map host pages for integrated graphics while
> > > + * extending SVM support there.
> > > + */
> > > + for (i = 0; i < npages; i++) {
> > > + u64 addr = range->pfns[i] & ~((1UL << range->pfn_shift) - 1);
> > > +
> > > + if (sg && (addr == (sg_dma_address(sg) + sg->length))) {
> > > + sg->length += PAGE_SIZE;
> > > + sg_dma_len(sg) += PAGE_SIZE;
> > > + continue;
> > > + }
> > > +
> > > + if (sg)
> > > + sg_page_sizes |= sg->length;
> > > +
> > > + sg = sg ? __sg_next(sg) : st->sgl;
> > > + sg_dma_address(sg) = addr;
> > > + sg_dma_len(sg) = PAGE_SIZE;
> >
> > This still can't be like this - assigning pfn to 'dma_address' is
> > fundamentally wrong.
> >
> > Whatever explanation you had, this needs to be fixed up first before we get
> > to this patch.
> >
>
> The pfn is converted into a device address which goes into sg_dma_address.
> Ok, let me think about what else we can do here.
If you combine this with the other function and make it so only
DEVICE_PRIVATE pages get converted toa dma_address with out dma_map,
then that would make sense.
> > > +static int
> > > +i915_svm_invalidate_range_start(struct mmu_notifier *mn,
> > > + const struct mmu_notifier_range *update)
> > > +{
> > > + struct i915_svm *svm = container_of(mn, struct i915_svm, notifier);
> > > + unsigned long length = update->end - update->start;
> > > +
> > > + DRM_DEBUG_DRIVER("start 0x%lx length 0x%lx\n", update->start, length);
> > > + if (!mmu_notifier_range_blockable(update))
> > > + return -EAGAIN;
> > > +
> > > + i915_gem_vm_unbind_svm_buffer(svm->vm, update->start, length);
> > > + return 0;
> > > +}
> >
> > I still think you should strive for a better design than putting a
> > notifier across the entire address space..
> >
>
> Yah, thought it could be later optimization.
> If I think about it, it has be be a new user API to set the range,
> or an intermediate data structure for tracking the bound ranges.
> Will look into it.
Well, there are lots of options. Like I said, implicit ODP uses a
level of the device page table to attach the notifier.
There are many performance trade offs here, it depends what works best
for your work load I suppose. But usually the fault path is the fast
thing, so I would think to avoid registering mmu_intervals on it and
accept the higher probability of collisions.
Jason
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