docs: kvm: vfio: Suggest KVM_DEV_VFIO_GROUP_ADD vs VFIO_GROUP_GET_DEVICE_FD ordering

as some vfio_device's open_device op requires kvm pointer and kvm pointer
set is part of GROUP_ADD.

Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Yi Liu <yi.l.liu@intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/20230421053611.55839-1-yi.l.liu@intel.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

diff --git a/Documentation/virt/kvm/devices/vfio.rst b/Documentation/virt/kvm/devices/vfio.rst
index 2d20dc5..08b5442 100644 (file)
--- a/Documentation/virt/kvm/devices/vfio.rst
+++ b/Documentation/virt/kvm/devices/vfio.rst
@@ -39,3 +39,8 @@ KVM_DEV_VFIO_GROUP attributes:
        - @groupfd is a file descriptor for a VFIO group;
        - @tablefd is a file descriptor for a TCE table allocated via
          KVM_CREATE_SPAPR_TCE.
+
+The GROUP_ADD operation above should be invoked prior to accessing the
+device file descriptor via VFIO_GROUP_GET_DEVICE_FD in order to support
+drivers which require a kvm pointer to be set in their .open_device()
+callback.