Re: [RFC v2 05/12] drm/i915/svm: Page table mirroring support

[Jason Gunthorpe <jgg@mellanox.com>]

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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