[PATCH v2 3/3] drm/xe: Get page on user fence creation

[Matthew Brost <matthew.brost@intel.com>]

Attempt to get page on user fence creation and kmap_local_page on
signaling. Should reduce latency and can ensure 64 bit atomicity
compared to copy_to_user.

v2:
 - Prefault page and drop ref (Thomas)
 - Use set_page_dirty_lock (Thomas)
 - try_cmpxchg64 loop (Thomas)

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
---
 drivers/gpu/drm/xe/xe_sync.c | 52 +++++++++++++++++++++++++++++++-----
 1 file changed, 45 insertions(+), 7 deletions(-)

diff --git a/drivers/gpu/drm/xe/xe_sync.c b/drivers/gpu/drm/xe/xe_sync.c
index c20e1f9ad267..bf7f22519cc5 100644
--- a/drivers/gpu/drm/xe/xe_sync.c
+++ b/drivers/gpu/drm/xe/xe_sync.c
@@ -6,6 +6,7 @@
 #include "xe_sync.h"
 
 #include <linux/dma-fence-array.h>
+#include <linux/highmem.h>
 #include <linux/kthread.h>
 #include <linux/sched/mm.h>
 #include <linux/uaccess.h>
@@ -28,6 +29,7 @@ struct xe_user_fence {
 	u64 __user *addr;
 	u64 value;
 	int signalled;
+	bool use_page;
 };
 
 static void user_fence_destroy(struct kref *kref)
@@ -53,7 +55,9 @@ static struct xe_user_fence *user_fence_create(struct xe_device *xe, u64 addr,
 					       u64 value)
 {
 	struct xe_user_fence *ufence;
+	struct page *page;
 	u64 __user *ptr = u64_to_user_ptr(addr);
+	int ret;
 
 	if (!access_ok(ptr, sizeof(ptr)))
 		return ERR_PTR(-EFAULT);
@@ -69,19 +73,53 @@ static struct xe_user_fence *user_fence_create(struct xe_device *xe, u64 addr,
 	ufence->mm = current->mm;
 	mmgrab(ufence->mm);
 
+	/* Prefault page */
+	ret = get_user_pages_fast(addr, 1, FOLL_WRITE, &page);
+	if (ret == 1) {
+		ufence->use_page = true;
+		put_page(page);
+	}
+
 	return ufence;
 }
 
 static void user_fence_worker(struct work_struct *w)
 {
 	struct xe_user_fence *ufence = container_of(w, struct xe_user_fence, worker);
-
-	if (mmget_not_zero(ufence->mm)) {
-		kthread_use_mm(ufence->mm);
-		if (copy_to_user(ufence->addr, &ufence->value, sizeof(ufence->value)))
-			XE_WARN_ON("Copy to user failed");
-		kthread_unuse_mm(ufence->mm);
-		mmput(ufence->mm);
+	struct mm_struct *mm = ufence->mm;
+
+	if (mmget_not_zero(mm)) {
+		kthread_use_mm(mm);
+		if (ufence->use_page) {
+			struct page *page;
+			int ret;
+
+			ret = get_user_pages_fast((unsigned long)ufence->addr,
+						  1, FOLL_WRITE, &page);
+			if (ret == 1) {
+				u64 *ptr;
+				u64 old = 0;
+				void *va;
+
+				va = kmap_local_page(page);
+				ptr = va + offset_in_page(ufence->addr);
+				while (!try_cmpxchg64(ptr, &old, ufence->value))
+					continue;
+				kunmap_local(va);
+
+				set_page_dirty_lock(page);
+				put_page(page);
+			} else {
+				ufence->use_page = false;
+			}
+		}
+		if (!ufence->use_page) {
+			if (copy_to_user(ufence->addr, &ufence->value,
+					 sizeof(ufence->value)))
+				drm_warn(&ufence->xe->drm, "Copy to user failed\n");
+		}
+		kthread_unuse_mm(mm);
+		mmput(mm);
 	}
 
 	wake_up_all(&ufence->xe->ufence_wq);
-- 
2.34.1