[RFC 06/13] drm/i915/svm: Page table mirroring support

[Niranjan Vishwanathapura]

On Tue, Nov 26, 2019 at 06:32:52PM +0000, Jason Gunthorpe wrote:
>On Mon, Nov 25, 2019 at 11:33:27AM -0500, Jerome Glisse wrote:
>> On Fri, Nov 22, 2019 at 11:33:12PM +0000, Jason Gunthorpe wrote:
>> > On Fri, Nov 22, 2019 at 12:57:27PM -0800, Niranjana Vishwanathapura wrote:
>>
>> [...]
>>
>> > > +static int
>> > > +i915_range_fault(struct i915_svm *svm, struct hmm_range *range)
>> > > +{
>> > > +	long ret;
>> > > +
>> > > +	range->default_flags = 0;
>> > > +	range->pfn_flags_mask = -1UL;
>> > > +
>> > > +	ret = hmm_range_register(range, &svm->mirror);
>> > > +	if (ret) {
>> > > +		up_read(&svm->mm->mmap_sem);
>> > > +		return (int)ret;
>> > > +	}
>> >
>> >
>> > Using a temporary range is the pattern from nouveau, is it really
>> > necessary in this driver?
>>
>> Just to comment on this, for GPU the usage model is not application
>> register range of virtual address it wants to use. It is GPU can
>> access _any_ CPU valid address just like the CPU would (modulo mmap
>> of device file).
>>
>> This is because the API you want in userspace is application passing
>> random pointer to the GPU and GPU being able to chase down any kind
>> of random pointer chain (assuming all valid ie pointing to valid
>> virtual address for the process).
>>
>> This is unlike the RDMA case.
>
>No, RDMA has the full address space option as well, it is called
>'implicit ODP'
>
>This is implemented by registering ranges at a level in our page
>table (currently 512G) whenever that level has populated pages below
>it.
>
>I think is a better strategy than temporary ranges.
>
>But other GPU drivers like AMD are using BO and TTM objects with fixed
>VA ranges and the range is tied to the BO/TTM.
>
>I'm not sure if this i915 usage is closer to AMD or closer to nouveau.
>

I don't know the full details of the HMM usecases in amd and nouveau.
AMD seems to be using it for usrptr objects which is tied to a BO.
I am not sure if nouveau has any BO tied to these address ranges.

Niranjana

>Jason