[PATCH v6 2/3] vfio: add bus master feature to device feature ioctl

[Nipun Gupta <nipun.gupta@amd.com>]

add bus mastering control to VFIO_DEVICE_FEATURE IOCTL. The VFIO user
can use this feature to enable or disable the Bus Mastering of a
device bound to VFIO.

Co-developed-by: Shubham Rohila <shubham.rohila@amd.com>
Signed-off-by: Shubham Rohila <shubham.rohila@amd.com>
Signed-off-by: Nipun Gupta <nipun.gupta@amd.com>
---

Changes v5->v6:
- Updated description of VFIO_DEVICE_FEATURE_BUS_MASTER IOCTL
- Used 0 for clear and 1 for set bus master operation

Changes in v5:
- This patch is newly added which proposes a new flag
  VFIO_DEVICE_FEATURE_BUS_MASTER in VFIO_DEVICE_FEATURE IOCTL.

include/uapi/linux/vfio.h | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
index 20c804bdc09c..6858cd80d597 100644
--- a/include/uapi/linux/vfio.h
+++ b/include/uapi/linux/vfio.h
@@ -1287,6 +1287,27 @@ struct vfio_device_feature_mig_data_size {
 
 #define VFIO_DEVICE_FEATURE_MIG_DATA_SIZE 9
 
+/**
+ * Upon VFIO_DEVICE_FEATURE_SET, set or clear the BUS mastering for the device
+ * based on the operation specified in op flag.
+ *
+ * The functionality is incorporated for devices that needs bus master control,
+ * but the in-band device interface lacks the support. Consequently, it is not
+ * applicable to PCI devices, as bus master control for PCI devices is managed
+ * in-band through the configuration space. At present, this feature is supported
+ * only for CDX devices.
+ * When the device's BUS MASTER setting is configured as CLEAR, it will result in
+ * blocking all incoming DMA requests from the device. On the other hand, configuring
+ * the device's BUS MASTER setting as SET (enable) will grant the device the
+ * capability to perform DMA to the host memory.
+ */
+struct vfio_device_feature_bus_master {
+	__u32 op;
+#define		VFIO_DEVICE_FEATURE_CLEAR_MASTER	0	/* Clear Bus Master */
+#define		VFIO_DEVICE_FEATURE_SET_MASTER		1	/* Set Bus Master */
+};
+#define VFIO_DEVICE_FEATURE_BUS_MASTER 10
+
 /* -------- API for Type1 VFIO IOMMU -------- */
 
 /**
-- 
2.17.1