Re: [Intel-gfx] [PATCH] drm/i915: Refine VT-d scanout workaround

[Chris Wilson <chris@chris-wilson.co.uk>]

Quoting Matthew Auld (2021-02-11 14:25:41)
> On 10/02/2021 23:39, Chris Wilson wrote:
> > VT-d may cause overfetch of the scanout PTE, both before and after the
> > vma (depending on the scanout orientation). bspec recommends that we
> > provide a tile-row in either directions, and suggests using 160 PTE,
> > warning that the accesses will wrap around the ends of the GGTT.
> > Currently, we fill the entire GGTT with scratch pages when using VT-d to
> > always ensure there are valid entries around every vma, including
> > scanout. However, writing every PTE is slow as on recent devices we
> > perform 8MiB of uncached writes, incurring an extra 100ms during resume.
> > 
> > If instead we focus on only putting guard pages around scanout, we can
> > avoid touching the whole GGTT. To avoid having to introduce extra nodes
> > around each scanout vma, we adjust the scanout drm_mm_node to be smaller
> > than the allocated space, and fixup the extra PTE during dma binding.
> > 
> > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> > Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > Cc: Matthew Auld <matthew.auld@intel.com>
> > ---
> >   drivers/gpu/drm/i915/gem/i915_gem_domain.c |  3 ++
> >   drivers/gpu/drm/i915/gt/intel_ggtt.c       | 37 ++++++++--------------
> >   drivers/gpu/drm/i915/i915_gem_gtt.h        |  1 +
> >   drivers/gpu/drm/i915/i915_vma.c            | 23 ++++++++++++++
> >   drivers/gpu/drm/i915/i915_vma_types.h      |  1 +
> >   5 files changed, 41 insertions(+), 24 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/gem/i915_gem_domain.c b/drivers/gpu/drm/i915/gem/i915_gem_domain.c
> > index 0478b069c202..9f2ccc255ca1 100644
> > --- a/drivers/gpu/drm/i915/gem/i915_gem_domain.c
> > +++ b/drivers/gpu/drm/i915/gem/i915_gem_domain.c
> > @@ -345,6 +345,9 @@ i915_gem_object_pin_to_display_plane(struct drm_i915_gem_object *obj,
> >       if (ret)
> >               goto err;
> >   
> > +     if (intel_scanout_needs_vtd_wa(i915))
> > +             flags |= PIN_VTD;
> > +
> >       /*
> >        * As the user may map the buffer once pinned in the display plane
> >        * (e.g. libkms for the bootup splash), we have to ensure that we
> > diff --git a/drivers/gpu/drm/i915/gt/intel_ggtt.c b/drivers/gpu/drm/i915/gt/intel_ggtt.c
> > index b0b8ded834f0..416f77f48561 100644
> > --- a/drivers/gpu/drm/i915/gt/intel_ggtt.c
> > +++ b/drivers/gpu/drm/i915/gt/intel_ggtt.c
> > @@ -238,6 +238,11 @@ static void gen8_ggtt_insert_entries(struct i915_address_space *vm,
> >   
> >       gte = (gen8_pte_t __iomem *)ggtt->gsm;
> >       gte += vma->node.start / I915_GTT_PAGE_SIZE;
> > +
> > +     end = gte - vma->guard / I915_GTT_PAGE_SIZE;
> > +     while (end < gte)
> > +             gen8_set_pte(end++, vm->scratch[0]->encode);
> > +
> >       end = gte + vma->node.size / I915_GTT_PAGE_SIZE;
> >   
> >       for_each_sgt_daddr(addr, iter, vma->pages)
> > @@ -245,6 +250,7 @@ static void gen8_ggtt_insert_entries(struct i915_address_space *vm,
> >       GEM_BUG_ON(gte > end);
> >   
> >       /* Fill the allocated but "unused" space beyond the end of the buffer */
> > +     end += vma->guard / I915_GTT_PAGE_SIZE;
> >       while (gte < end)
> >               gen8_set_pte(gte++, vm->scratch[0]->encode);
> >   
> > @@ -289,6 +295,11 @@ static void gen6_ggtt_insert_entries(struct i915_address_space *vm,
> >   
> >       gte = (gen6_pte_t __iomem *)ggtt->gsm;
> >       gte += vma->node.start / I915_GTT_PAGE_SIZE;
> > +
> > +     end = gte - vma->guard / I915_GTT_PAGE_SIZE;
> > +     while (end < gte)
> > +             gen8_set_pte(end++, vm->scratch[0]->encode);
> > +
> >       end = gte + vma->node.size / I915_GTT_PAGE_SIZE;
> >   
> >       for_each_sgt_daddr(addr, iter, vma->pages)
> > @@ -296,6 +307,7 @@ static void gen6_ggtt_insert_entries(struct i915_address_space *vm,
> >       GEM_BUG_ON(gte > end);
> >   
> >       /* Fill the allocated but "unused" space beyond the end of the buffer */
> > +     end += vma->guard / I915_GTT_PAGE_SIZE;
> >       while (gte < end)
> >               iowrite32(vm->scratch[0]->encode, gte++);
> >   
> > @@ -311,27 +323,6 @@ static void nop_clear_range(struct i915_address_space *vm,
> >   {
> >   }
> >   
> > -static void gen8_ggtt_clear_range(struct i915_address_space *vm,
> > -                               u64 start, u64 length)
> > -{
> > -     struct i915_ggtt *ggtt = i915_vm_to_ggtt(vm);
> > -     unsigned int first_entry = start / I915_GTT_PAGE_SIZE;
> > -     unsigned int num_entries = length / I915_GTT_PAGE_SIZE;
> > -     const gen8_pte_t scratch_pte = vm->scratch[0]->encode;
> > -     gen8_pte_t __iomem *gtt_base =
> > -             (gen8_pte_t __iomem *)ggtt->gsm + first_entry;
> > -     const int max_entries = ggtt_total_entries(ggtt) - first_entry;
> > -     int i;
> > -
> > -     if (WARN(num_entries > max_entries,
> > -              "First entry = %d; Num entries = %d (max=%d)\n",
> > -              first_entry, num_entries, max_entries))
> > -             num_entries = max_entries;
> > -
> > -     for (i = 0; i < num_entries; i++)
> > -             gen8_set_pte(&gtt_base[i], scratch_pte);
> > -}
> > -
> >   static void bxt_vtd_ggtt_wa(struct i915_address_space *vm)
> >   {
> >       /*
> > @@ -898,8 +889,6 @@ static int gen8_gmch_probe(struct i915_ggtt *ggtt)
> >       ggtt->vm.cleanup = gen6_gmch_remove;
> >       ggtt->vm.insert_page = gen8_ggtt_insert_page;
> >       ggtt->vm.clear_range = nop_clear_range;
> > -     if (intel_scanout_needs_vtd_wa(i915))
> > -             ggtt->vm.clear_range = gen8_ggtt_clear_range;
> >   
> >       ggtt->vm.insert_entries = gen8_ggtt_insert_entries;
> >   
> > @@ -1045,7 +1034,7 @@ static int gen6_gmch_probe(struct i915_ggtt *ggtt)
> >       ggtt->vm.alloc_pt_dma = alloc_pt_dma;
> >   
> >       ggtt->vm.clear_range = nop_clear_range;
> > -     if (!HAS_FULL_PPGTT(i915) || intel_scanout_needs_vtd_wa(i915))
> > +     if (!HAS_FULL_PPGTT(i915))
> >               ggtt->vm.clear_range = gen6_ggtt_clear_range;
> >       ggtt->vm.insert_page = gen6_ggtt_insert_page;
> >       ggtt->vm.insert_entries = gen6_ggtt_insert_entries;
> > diff --git a/drivers/gpu/drm/i915/i915_gem_gtt.h b/drivers/gpu/drm/i915/i915_gem_gtt.h
> > index c9b0ee5e1d23..8a2dfc7144cf 100644
> > --- a/drivers/gpu/drm/i915/i915_gem_gtt.h
> > +++ b/drivers/gpu/drm/i915/i915_gem_gtt.h
> > @@ -41,6 +41,7 @@ int i915_gem_gtt_insert(struct i915_address_space *vm,
> >   #define PIN_HIGH            BIT_ULL(5)
> >   #define PIN_OFFSET_BIAS             BIT_ULL(6)
> >   #define PIN_OFFSET_FIXED    BIT_ULL(7)
> > +#define PIN_VTD                      BIT_ULL(8)
> >   
> >   #define PIN_GLOBAL          BIT_ULL(10) /* I915_VMA_GLOBAL_BIND */
> >   #define PIN_USER            BIT_ULL(11) /* I915_VMA_LOCAL_BIND */
> > diff --git a/drivers/gpu/drm/i915/i915_vma.c b/drivers/gpu/drm/i915/i915_vma.c
> > index caa9b041616b..dccd36ff1a6d 100644
> > --- a/drivers/gpu/drm/i915/i915_vma.c
> > +++ b/drivers/gpu/drm/i915/i915_vma.c
> > @@ -38,6 +38,8 @@
> >   #include "i915_trace.h"
> >   #include "i915_vma.h"
> >   
> > +#define VTD_GUARD roundup_pow_of_two(160 * SZ_4K) /* 160 PTE padding */
> > +
> >   static struct i915_global_vma {
> >       struct i915_global base;
> >       struct kmem_cache *slab_vmas;
> > @@ -552,6 +554,9 @@ bool i915_vma_misplaced(const struct i915_vma *vma,
> >           vma->node.start != (flags & PIN_OFFSET_MASK))
> >               return true;
> >   
> > +     if (flags & PIN_VTD && vma->guard < VTD_GUARD)
> > +             return true;
> > +
> >       return false;
> >   }
> >   
> > @@ -637,6 +642,13 @@ i915_vma_insert(struct i915_vma *vma, u64 size, u64 alignment, u64 flags)
> >                                 alignment, vma->fence_alignment);
> >       }
> >   
> > +     /* VT-d requires padding before/after the vma */
> > +     if (flags & PIN_VTD) {
> > +             alignment = max_t(typeof(alignment), alignment, VTD_GUARD);
> > +             vma->guard = alignment;
> > +             size += 2 * vma->guard;
> > +     }
> > +
> >       GEM_BUG_ON(!IS_ALIGNED(size, I915_GTT_PAGE_SIZE));
> >       GEM_BUG_ON(!IS_ALIGNED(alignment, I915_GTT_MIN_ALIGNMENT));
> >       GEM_BUG_ON(!is_power_of_2(alignment));
> > @@ -725,6 +737,11 @@ i915_vma_insert(struct i915_vma *vma, u64 size, u64 alignment, u64 flags)
> >   
> >       list_add_tail(&vma->vm_link, &vma->vm->bound_list);
> >   
> > +     if (flags & PIN_VTD) {
> > +             vma->node.start += vma->guard;
> > +             vma->node.size -= 2 * vma->guard;
> > +     }
> > +
> 
> So we adjust the node to be twice as large, reserve it in the mm, and 
> then shrink it again here. Seems a little scary to modify 
> node.start/node.size after insertion?

Definitely scary. So far it feels like the most viable approach. At
least the concept of including a red-zone around the vma/drm_mm_node as
part of that structure seems useful, and familiar.
 
> Just wondering if this might upset something like evict_for_node, where 
> we inspect the allocated nodes over some range, rather than holes, and 
> since we shrunk the node it might get confused into thinking the padding 
> is actually free space. Or maybe that doesn't really matter much for the 
> GGTT?

The stumbling block I had earlier then forgot about is the drm_mm_node's
computed hole_size uses the adjacent drm_mm_node.start. So as we've
adjusted the node.start, it will later on declare a bigger hole and
reuse part of our guard pages. That we will then clobber on resume.

(There's also consequences for the address interval tree, but that will
continue to work correctly as we won't move the node.start beyond the
allocated space.)

Hmm. Fortunately, we have wrapped all users of vma->node.start, at least
for ggtt, with i915_ggtt_offset(). So it is feasible to adjust the
offset by vma->guard there.

Let's try that.
-Chris
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