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

[Jason Gunthorpe <jgg@mellanox.com>]

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.

Jason
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