[Intel-gfx] [PATCH 06/11] drm/i915/gtt: Convert vm->scratch into an array

[Chris Wilson]

Quoting Mika Kuoppala (2019-07-10 15:18:32)
> Chris Wilson <chris at chris-wilson.co.uk> writes:
> > -     if (i915_vm_is_4lvl(vm)) {
> > -             if (unlikely(setup_page_dma(vm, &vm->scratch_pdp))) {
> > -                     ret = -ENOMEM;
> > -                     goto free_pd;
> > -             }
> > -             fill_page_dma(&vm->scratch_pdp,
> > -                           gen8_pde_encode(vm->scratch_pdp.daddr,
> > -                                           I915_CACHE_LLC));
> > +             fill_px(&vm->scratch[i], vm->scratch[i - 1].encode);
> > +             vm->scratch[i].encode =
> > +                     gen8_pde_encode(px_dma(&vm->scratch[i]),
> > +                                     I915_CACHE_LLC);
> 
> Ok. The new code makes perfect sense here.
> 
> And with it confusion arises: how did we manage
> to get the old code work with pdp encoding pointing to itself?

What the.... You're right. That must have caused some funky GPU hangs if
people tried to access something far outside of their set.

Hmm, you know that's exactly what live_contexts/vm_isolation tries. Well
it tries to write into random invalid addresses and see if the writes
affect scratch of another context. Did I choose randomly carefully
enough? Hmm. offset &= -sizeof(u32); Is there a danger there that's only
u32 and not u64. Maybe. But otherwise it looks like it should be picking
a prng over the whole vm->total and so should be tripping over the
recursion :|
-Chris