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

[Jason Gunthorpe <jgg@mellanox.com>]

On Wed, Dec 04, 2019 at 04:51:36PM -0500, Jerome Glisse wrote:
> On Tue, Dec 03, 2019 at 11:19:43AM -0800, Niranjan Vishwanathapura wrote:
> > 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.
> 
> HMM original design did not have range and was well suited to nouveau.
> Recent changes make it more tie to the range and less suited to nouveau.
> I would not consider 512GB implicit range as good thing. Plan i have is
> to create implicit range and align them on vma. I do not know when i will
> have time to get to that.

For mlx5 the 512G is aligned to the page table levels, so it is a
reasonable approximation. GPU could do the same.

Not sure VMA related is really any better..

Jason
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