[RFC PATCH] drm/xe/bo: Honor madvise(2) advices

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

The user can give advices as to how the CPU will access an
address range. Use those advices to determine the number of
bo pages to prefault on a page-fault.

Do this regardless of whether we can find a way to avoid the
fairly slow vm_insert_pfn_prot() to populate buffer
object maps.

Initially, fault up to 512 pages on sequential access and
a single page on random access.

Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
---
 drivers/gpu/drm/xe/xe_bo.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/xe/xe_bo.c b/drivers/gpu/drm/xe/xe_bo.c
index 6fd6ce6c6586..07d0d954f826 100644
--- a/drivers/gpu/drm/xe/xe_bo.c
+++ b/drivers/gpu/drm/xe/xe_bo.c
@@ -1821,15 +1821,31 @@ static int xe_bo_fault_migrate(struct xe_bo *bo, struct ttm_operation_ctx *ctx,
 	return err;
 }
 
+/*
+ * Number of prefaulted pages for the MADV_SEQUENTIAL and
+ * MADV_RANDOM madvise() advices.
+ */
+#define XE_BO_VM_NUM_PREFAULT_SEQ  512
+#define XE_BO_VM_NUM_PREFAULT_RAND 1
+
 /* Call into TTM to populate PTEs, and register bo for PTE removal on runtime suspend. */
 static vm_fault_t __xe_bo_cpu_fault(struct vm_fault *vmf, struct xe_device *xe, struct xe_bo *bo)
 {
+	const struct vm_area_struct *vma = vmf->vma;
+	pgoff_t num_prefault;
 	vm_fault_t ret;
 
 	trace_xe_bo_cpu_fault(bo);
 
+	if (vma->vm_flags & VM_SEQ_READ)
+		num_prefault = XE_BO_VM_NUM_PREFAULT_SEQ;
+	else if (vma->vm_flags & VM_RAND_READ)
+		num_prefault = XE_BO_VM_NUM_PREFAULT_RAND;
+	else
+		num_prefault = TTM_BO_VM_NUM_PREFAULT;
+
 	ret = ttm_bo_vm_fault_reserved(vmf, vmf->vma->vm_page_prot,
-				       TTM_BO_VM_NUM_PREFAULT);
+				       num_prefault);
 	/*
 	 * When TTM is actually called to insert PTEs, ensure no blocking conditions
 	 * remain, in which case TTM may drop locks and return VM_FAULT_RETRY.
-- 
2.51.1