Currently, existing VFIO_MIG_GET_PRECOPY_INFO implementations don't
assign info.flags before copy_to_user().

Because they copy the struct in from userspace first, this effectively
echoes userspace-provided flags back as output, preventing the field
from being used to report new reliable data from the drivers.

Add support for a new device feature named
VFIO_DEVICE_FEATURE_MIG_PRECOPY_INFOv2.

On SET, enables the v2 pre_copy_info behaviour, where the
vfio_precopy_info.flags is a valid output field.

Signed-off-by: Yishai Hadas <yishaih@nvidia.com>
---
 drivers/vfio/pci/vfio_pci_core.c |  1 +
 drivers/vfio/vfio_main.c         | 20 ++++++++++++++++++++
 include/linux/vfio.h             |  1 +
 3 files changed, 22 insertions(+)

diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
index d43745fe4c84..e22280f53ebf 100644
--- a/drivers/vfio/pci/vfio_pci_core.c
+++ b/drivers/vfio/pci/vfio_pci_core.c
@@ -736,6 +736,7 @@ void vfio_pci_core_close_device(struct vfio_device *core_vdev)
 #endif
 	vfio_pci_core_disable(vdev);
 
+	core_vdev->precopy_info_flags_fix = 0;
 	vfio_pci_dma_buf_cleanup(vdev);
 
 	mutex_lock(&vdev->igate);
diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
index 742477546b15..2243a6eb5547 100644
--- a/drivers/vfio/vfio_main.c
+++ b/drivers/vfio/vfio_main.c
@@ -964,6 +964,23 @@ vfio_ioctl_device_feature_migration_data_size(struct vfio_device *device,
 	return 0;
 }
 
+static int
+vfio_ioctl_device_feature_migration_precopy_info_v2(struct vfio_device *device,
+						    u32 flags, size_t argsz)
+{
+	int ret;
+
+	if (!(device->migration_flags & VFIO_MIGRATION_PRE_COPY))
+		return -EINVAL;
+
+	ret = vfio_check_feature(flags, argsz, VFIO_DEVICE_FEATURE_SET, 0);
+	if (ret != 1)
+		return ret;
+
+	device->precopy_info_flags_fix = 1;
+	return 0;
+}
+
 static int vfio_ioctl_device_feature_migration(struct vfio_device *device,
 					       u32 flags, void __user *arg,
 					       size_t argsz)
@@ -1251,6 +1268,9 @@ static int vfio_ioctl_device_feature(struct vfio_device *device,
 		return vfio_ioctl_device_feature_migration_data_size(
 			device, feature.flags, arg->data,
 			feature.argsz - minsz);
+	case VFIO_DEVICE_FEATURE_MIG_PRECOPY_INFOv2:
+		return vfio_ioctl_device_feature_migration_precopy_info_v2(
+			device, feature.flags, feature.argsz - minsz);
 	default:
 		if (unlikely(!device->ops->device_feature))
 			return -ENOTTY;
diff --git a/include/linux/vfio.h b/include/linux/vfio.h
index e90859956514..3ff21374aeee 100644
--- a/include/linux/vfio.h
+++ b/include/linux/vfio.h
@@ -52,6 +52,7 @@ struct vfio_device {
 	struct vfio_device_set *dev_set;
 	struct list_head dev_set_list;
 	unsigned int migration_flags;
+	u8 precopy_info_flags_fix;
 	struct kvm *kvm;
 
 	/* Members below here are private, not for driver use */
-- 
2.18.1