vfio/pci: Add ioeventfd support

The ioeventfd here is actually irqfd handling of an ioeventfd such as
supported in KVM.  A user is able to pre-program a device write to
occur when the eventfd triggers.  This is yet another instance of
eventfd-irqfd triggering between KVM and vfio.  The impetus for this
is high frequency writes to pages which are virtualized in QEMU.
Enabling this near-direct write path for selected registers within
the virtualized page can improve performance and reduce overhead.
Specifically this is initially targeted at NVIDIA graphics cards where
the driver issues a write to an MMIO register within a virtualized
region in order to allow the MSI interrupt to re-trigger.

Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

include/uapi/linux/vfio.h

@@ -575,6 +575,33 @@ struct vfio_device_gfx_plane_info {
 
 #define VFIO_DEVICE_GET_GFX_DMABUF _IO(VFIO_TYPE, VFIO_BASE + 15)
 
+/**
+ * VFIO_DEVICE_IOEVENTFD - _IOW(VFIO_TYPE, VFIO_BASE + 16,
+ *                              struct vfio_device_ioeventfd)
+ *
+ * Perform a write to the device at the specified device fd offset, with
+ * the specified data and width when the provided eventfd is triggered.
+ * vfio bus drivers may not support this for all regions, for all widths,
+ * or at all.  vfio-pci currently only enables support for BAR regions,
+ * excluding the MSI-X vector table.
+ *
+ * Return: 0 on success, -errno on failure.
+ */
+struct vfio_device_ioeventfd {
+	__u32	argsz;
+	__u32	flags;
+#define VFIO_DEVICE_IOEVENTFD_8		(1 << 0) /* 1-byte write */
+#define VFIO_DEVICE_IOEVENTFD_16	(1 << 1) /* 2-byte write */
+#define VFIO_DEVICE_IOEVENTFD_32	(1 << 2) /* 4-byte write */
+#define VFIO_DEVICE_IOEVENTFD_64	(1 << 3) /* 8-byte write */
+#define VFIO_DEVICE_IOEVENTFD_SIZE_MASK	(0xf)
+	__u64	offset;			/* device fd offset of write */
+	__u64	data;			/* data to be written */
+	__s32	fd;			/* -1 for de-assignment */
+};
+
+#define VFIO_DEVICE_IOEVENTFD		_IO(VFIO_TYPE, VFIO_BASE + 16)
+
 /* -------- API for Type1 VFIO IOMMU -------- */
 
 /**