[PATCH 2/2] drm/i915/gvt: subsitute kvm_read/write_guest with vfio_iova_rw

[Yan Zhao]

As a device model, it is better to read/write guest memory using vfio
interface, so that vfio is able to maintain dirty info of device IOVAs.

Compared to CPU side interfaces kvm_read/write_guest(), vfio_iova_rw()
has ~600 cycles more overhead on average.
-------------------------------------
|    interface     | avg cpu cycles |
|-----------------------------------|
| kvm_write_guest  |     1546       |
| ----------------------------------|
| kvm_read_guest   |     686        |
|-----------------------------------|
| vfio_iova_rw(w)  |     2233       |
|-----------------------------------|
| vfio_iova_rw(r)  |     1262       |
-------------------------------------

Comparison of benchmarks scores are as blow:
---------------------------------------------------------
|  avg score  | kvm_read/write_guest   | vfio_iova_rw   |
---------------------------------------------------------
|   Glmark2   |         1132           |      1138.2    |
---------------------------------------------------------
|  Lightsmark |        61.558          |      61.538    |
|--------------------------------------------------------
|  OpenArena  |        142.77          |      136.6     |
---------------------------------------------------------
|   Heaven    |         698            |      686.8     |
--------------------------------------------------------
No obvious performance downgrade found.

Cc: Kevin Tian <kevin.tian@xxxxxxxxx>
Signed-off-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>
---
 drivers/gpu/drm/i915/gvt/kvmgt.c | 26 +++++++-------------------
 1 file changed, 7 insertions(+), 19 deletions(-)

diff --git a/drivers/gpu/drm/i915/gvt/kvmgt.c b/drivers/gpu/drm/i915/gvt/kvmgt.c
index bd79a9718cc7..576b05db3998 100644
--- a/drivers/gpu/drm/i915/gvt/kvmgt.c
+++ b/drivers/gpu/drm/i915/gvt/kvmgt.c
@@ -1966,31 +1966,19 @@ static int kvmgt_rw_gpa(unsigned long handle, unsigned long gpa,
 			void *buf, unsigned long len, bool write)
 {
 	struct kvmgt_guest_info *info;
-	struct kvm *kvm;
-	int idx, ret;
-	bool kthread = current->mm == NULL;
+	int ret;
+	struct intel_vgpu *vgpu;
+	struct device *dev;
 
 	if (!handle_valid(handle))
 		return -ESRCH;
 
 	info = (struct kvmgt_guest_info *)handle;
-	kvm = info->kvm;
-
-	if (kthread) {
-		if (!mmget_not_zero(kvm->mm))
-			return -EFAULT;
-		use_mm(kvm->mm);
-	}
-
-	idx = srcu_read_lock(&kvm->srcu);
-	ret = write ? kvm_write_guest(kvm, gpa, buf, len) :
-		      kvm_read_guest(kvm, gpa, buf, len);
-	srcu_read_unlock(&kvm->srcu, idx);
+	vgpu = info->vgpu;
+	dev = mdev_dev(vgpu->vdev.mdev);
 
-	if (kthread) {
-		unuse_mm(kvm->mm);
-		mmput(kvm->mm);
-	}
+	ret = write ? vfio_iova_rw(dev, gpa, buf, len, true) :
+			vfio_iova_rw(dev, gpa, buf, len, false);
 
 	return ret;
 }
-- 
2.17.1