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

[Niranjana Vishwanathapura]

On Fri, Dec 20, 2019 at 01:45:33PM +0000, Jason Gunthorpe wrote:
>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.
>

Ok thanks, will do that.

>> > > +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.
>

Ok thanks, yah, solution should tune for the performant path. Will look into it.

Thanks,
Niranjana

>Jason