On Wed, Feb 10, 2021 at 11:39:46PM +0000, 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@xxxxxxxxxxxxxxxxxx>
> Cc: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
> Cc: Matthew Auld <matthew.auld@xxxxxxxxx>
> ---
> 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(>t_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));
someh> @@ -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;
Was a bit worried for a second that this might give the display
a potentially misaligned vma start. But looks like you did consider
all that: VTD_GUARD==POT, alignment + guard both get bumped
to the max(). So AFAICS should guarantee everyone is happy.
I guess we're now wasting a lot more ggtt address space though?
Not sure if anyone has ever been at risk of running out though.
And DPT should help with this on new platforms.
> + vma->node.size -= 2 * vma->guard;
> + }
> +
> return 0;
> }
>
> @@ -734,6 +751,12 @@ i915_vma_detach(struct i915_vma *vma)
> GEM_BUG_ON(!drm_mm_node_allocated(&vma->node));
> GEM_BUG_ON(i915_vma_is_bound(vma, I915_VMA_GLOBAL_BIND | I915_VMA_LOCAL_BIND));
>
> + if (vma->guard) {
> + vma->node.start -= vma->guard;
> + vma->node.size += 2 * vma->guard;
> + vma->guard = 0;
> + }
> +
> /*
> * And finally now the object is completely decoupled from this
> * vma, we can drop its hold on the backing storage and allow
> diff --git a/drivers/gpu/drm/i915/i915_vma_types.h b/drivers/gpu/drm/i915/i915_vma_types.h
> index f5cb848b7a7e..bafec4e0b042 100644
> --- a/drivers/gpu/drm/i915/i915_vma_types.h
> +++ b/drivers/gpu/drm/i915/i915_vma_types.h
> @@ -190,6 +190,7 @@ struct i915_vma {
>
> u32 fence_size;
> u32 fence_alignment;
> + u32 guard;
>
> /**
> * Count of the number of times this vma has been opened by different
> --
> 2.20.1
--
Ville Syrjälä
Intel
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