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

["Thomas Hellström" <thomas.hellstrom@linux.intel.com>]

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.koenig@amd.com>
> > Cc: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
> > Cc: Matthew Auld <matthew.auld@intel.com>
> > Cc: Matthew Brost <matthew.brost@intel.com>
> > Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> > Signed-off-by: Nirmoy Das <nirmoy.das@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.

/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;
>