drm/i915/gvt: subsitute kvm_read/write_guest with vfio_dma_rw

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.

Cc: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200313031109.7989-1-yan.y.zhao@intel.com

drivers/gpu/drm/i915/gvt/kvmgt.c

@@ -2050,33 +2050,14 @@ 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;
 
 	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);
-
-	if (kthread) {
-		unuse_mm(kvm->mm);
-		mmput(kvm->mm);
-	}
-
-	return ret;
+	return vfio_dma_rw(kvmgt_vdev(info->vgpu)->vfio_group,
+			   gpa, buf, len, write);
 }
 
 static int kvmgt_read_gpa(unsigned long handle, unsigned long gpa,