Re: [PATCH 1/2] drm/xe/lnl: Only do gpu sys page clear for non-pooled BOs

[Nirmoy Das]

On 8/28/2024 10:12 AM, Thomas Hellström wrote:
On Wed, 2024-08-28 at 10:09 +0200, Thomas Hellström wrote:
On Wed, 2024-08-21 at 11:50 +0200, Nirmoy Das wrote:
Currently XE lacks clean-on-free implementation so using
TTM_TT_FLAG_CLEARED_ON_FREE is invalid. Remove usage of
TTM_TT_FLAG_CLEARED_ON_FREE and limit gpu system page clearing
only for WB cached BOs which are not pooled so there is no need to
return a zeroed pages to a pool.

Without the patch:
api_overhead_benchmark_l0 --testFilter=UsmMemoryAllocation:
UsmMemoryAllocation(api=l0 type=Host size=4KB) 79.439 us
UsmMemoryAllocation(api=l0 type=Host size=1GB) 98677.75 us
Perf tool top 5 entries:
11.16%  api_overhead_be [kernel.kallsyms] [k]
__pageblock_pfn_to_page
7.85%  api_overhead_be  [kernel.kallsyms] [k] cpa_flush
7.59%  api_overhead_be  [kernel.kallsyms] [k] find_next_iomem_res
7.24%  api_overhead_be  [kernel.kallsyms] [k] pages_are_mergeable
5.53%  api_overhead_be  [kernel.kallsyms] [k]
lookup_address_in_pgd_attr

With the patch:
UsmMemoryAllocation(api=l0 type=Host size=4KB) 78.164 us
UsmMemoryAllocation(api=l0 type=Host size=1GB) 98880.39 us
Perf tool top 5 entries:
25.40% api_overhead_be  [kernel.kallsyms] [k] clear_page_erms
9.89%  api_overhead_be  [kernel.kallsyms] [k] pages_are_mergeable
4.64%  api_overhead_be  [kernel.kallsyms] [k] cpa_flush
4.04%  api_overhead_be  [kernel.kallsyms] [k] find_next_iomem_res
3.96%  api_overhead_be  [kernel.kallsyms] [k] mod_find

This is still better than the base case where there was no
page clearing offloading.

Cc: Christian König <christian.koe...@amd.com>
Cc: Himal Prasad Ghimiray <himal.prasad.ghimi...@intel.com>
Cc: Matthew Auld <matthew.a...@intel.com>
Cc: Matthew Brost <matthew.br...@intel.com>
Cc: Thomas Hellström <thomas.hellst...@linux.intel.com>
Signed-off-by: Nirmoy Das <nirmoy....@intel.com>
Nirmoy, is it possible to split this up into a revert and then added
functionality so that we could quickly revert the security issue end
then add a separate patch that is an optimization?
Wait, I see now on the list there's already a revert patch. Let me
check what's actually pending review.


Yes, sent out https://patchwork.freedesktop.org/series/137855/ yesterday 
which should be easier

to review and merge. I will look into the page zero offload later on 
because for LNL we don't need to

clear CCS for WB and pool backed BOs doesn't have much alloc latency.


Regards,

Nirmoy


/Thomas


/Thomas

---
  drivers/gpu/drm/xe/xe_bo.c | 27 +++++++++++++++++++--------
  1 file changed, 19 insertions(+), 8 deletions(-)

diff --git a/drivers/gpu/drm/xe/xe_bo.c
b/drivers/gpu/drm/xe/xe_bo.c
index 6ed0e1955215..a18408d5d185 100644
--- a/drivers/gpu/drm/xe/xe_bo.c
+++ b/drivers/gpu/drm/xe/xe_bo.c
@@ -283,6 +283,7 @@ struct xe_ttm_tt {
        struct device *dev;
        struct sg_table sgt;
        struct sg_table *sg;
+       bool clear_system_pages;
  };
  
  static int xe_tt_map_sg(struct ttm_tt *tt)
@@ -397,12 +398,17 @@ static struct ttm_tt *xe_ttm_tt_create(struct
ttm_buffer_object *ttm_bo,
        }
  
  	/*
-        * If the device can support gpu clear system pages then
set
proper ttm
+        * If the device can support gpu clear system pages then
set
proper
         * flag. Zeroed pages are only required for
ttm_bo_type_device so
         * unwanted data is not leaked to userspace.
+        *
+        * XE currently does clear-on-alloc so gpu clear will only
work on
+        * non-pooled BO, DRM_XE_GEM_CPU_CACHING_WB otherwise
global
pool will
+        * get poisoned ono-zeroed pages.
         */
-       if (ttm_bo->type == ttm_bo_type_device && xe-
mem.gpu_page_clear_sys)
-               page_flags |= TTM_TT_FLAG_CLEARED_ON_FREE;
+       if (ttm_bo->type == ttm_bo_type_device && xe-
mem.gpu_page_clear_sys &&
+           bo->cpu_caching == DRM_XE_GEM_CPU_CACHING_WB)
+               tt->clear_system_pages = true;
  
  	err = ttm_tt_init(&tt->ttm, &bo->ttm, page_flags, caching,
extra_pages);
        if (err) {
@@ -416,8 +422,11 @@ static struct ttm_tt *xe_ttm_tt_create(struct
ttm_buffer_object *ttm_bo,
  static int xe_ttm_tt_populate(struct ttm_device *ttm_dev, struct
ttm_tt *tt,
                              struct ttm_operation_ctx *ctx)
  {
+       struct xe_ttm_tt *xe_tt;
        int err;
  
+	xe_tt = container_of(tt, struct xe_ttm_tt, ttm);
+
        /*
         * dma-bufs are not populated with pages, and the dma-
         * addresses are set up when moved to XE_PL_TT.
@@ -426,7 +435,7 @@ static int xe_ttm_tt_populate(struct ttm_device
*ttm_dev, struct ttm_tt *tt,
                return 0;
  
  	/* Clear TTM_TT_FLAG_ZERO_ALLOC when GPU is set to clear
system pages */
-       if (tt->page_flags & TTM_TT_FLAG_CLEARED_ON_FREE)
+       if (xe_tt->clear_system_pages)
                tt->page_flags &= ~TTM_TT_FLAG_ZERO_ALLOC;
  
  	err = ttm_pool_alloc(&ttm_dev->pool, tt, ctx);
@@ -664,6 +673,7 @@ static int xe_bo_move(struct ttm_buffer_object
*ttm_bo, bool evict,
        struct ttm_resource *old_mem = ttm_bo->resource;
        u32 old_mem_type = old_mem ? old_mem->mem_type :
XE_PL_SYSTEM;
        struct ttm_tt *ttm = ttm_bo->ttm;
+       struct xe_ttm_tt *xe_tt = container_of(ttm, struct
xe_ttm_tt, ttm);
        struct xe_migrate *migrate = NULL;
        struct dma_fence *fence;
        bool move_lacks_source;
@@ -671,15 +681,16 @@ static int xe_bo_move(struct
ttm_buffer_object
*ttm_bo, bool evict,
        bool needs_clear;
        bool handle_system_ccs = (!IS_DGFX(xe) &&
xe_bo_needs_ccs_pages(bo) &&
                                  ttm && ttm_tt_is_populated(ttm))
?
true : false;
-       bool clear_system_pages;
+       bool clear_system_pages = false;
        int ret = 0;
  
  	/*
         * Clear TTM_TT_FLAG_CLEARED_ON_FREE on bo creation path
when
         * moving to system as the bo doesn't have dma_mapping.
         */
-       if (!old_mem && ttm && !ttm_tt_is_populated(ttm))
-               ttm->page_flags &= ~TTM_TT_FLAG_CLEARED_ON_FREE;
+       if (!old_mem && ttm && !ttm_tt_is_populated(ttm) &&
+           xe_tt->clear_system_pages)
+               xe_tt->clear_system_pages = false;
  
  	/* Bo creation path, moving to system or TT. */
        if ((!old_mem && ttm) && !handle_system_ccs) {
@@ -703,7 +714,7 @@ static int xe_bo_move(struct ttm_buffer_object
*ttm_bo, bool evict,
        move_lacks_source = handle_system_ccs ? (!bo-
ccs_cleared)
:
                                                (!mem_type_is_vram
(o
ld_mem_type) && !tt_has_data);
  
-	clear_system_pages = ttm && (ttm->page_flags &
TTM_TT_FLAG_CLEARED_ON_FREE);
+       clear_system_pages = ttm && xe_tt->clear_system_pages;
        needs_clear = (ttm && ttm->page_flags &
TTM_TT_FLAG_ZERO_ALLOC) ||
                (!ttm && ttm_bo->type == ttm_bo_type_device) ||
                clear_system_pages;