[RFC PATCH 11/14] vfio: opt vfio-dev and VFIO group devices into sysfs_lazy

[Pavol Sakac <sakacpav@amazon.de>]

Call device_set_sysfs_lazy() on vfio_device->device (vfio-dev/) and
on the VFIO group's struct device (vfio/<group>/) before device_add().
With opt-in, vfio-dev directories materialise their attribute_groups
(vfio_dev_groups + drvdata->ops->dev_groups) on first access via the
sysfs lookup path instead of eagerly at device_add(). The VFIO
group device's flat attrs follow the same deferral.

Removes the eager creation cost on platforms with thousands of VFs
assigned to VFIO containers. No userspace ABI change.

Cc: Alex Williamson <alex.williamson@redhat.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: kvm@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Danilo Krummrich <dakr@kernel.org>
Cc: driver-core@lists.linux.dev
Cc: linux-api@vger.kernel.org
Assisted-by: Claude:claude-opus-4.7
Signed-off-by: Pavol Sakac <sakacpav@amazon.de>
---
 drivers/vfio/group.c     | 5 +++++
 drivers/vfio/vfio_main.c | 5 +++++
 2 files changed, 10 insertions(+)

diff --git a/drivers/vfio/group.c b/drivers/vfio/group.c
index b2299e5bc6df3..aa7e0033b96e8 100644
--- a/drivers/vfio/group.c
+++ b/drivers/vfio/group.c
@@ -533,6 +533,11 @@ static struct vfio_group *vfio_group_alloc(struct iommu_group *iommu_group,
 	group->dev.devt = MKDEV(MAJOR(vfio.group_devt), minor);
 	group->dev.class = &vfio_class;
 	group->dev.release = vfio_group_release;
+	/* Defer per-device attrs to first kernfs access; userspace reads via ioctl. */
+	if (device_set_sysfs_lazy(&group->dev)) {
+		put_device(&group->dev);
+		return ERR_PTR(-ENOMEM);
+	}
 	cdev_init(&group->cdev, &vfio_group_fops);
 	group->cdev.owner = THIS_MODULE;
 
diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
index 6222376ab6ab5..ab90279c44bd0 100644
--- a/drivers/vfio/vfio_main.c
+++ b/drivers/vfio/vfio_main.c
@@ -306,6 +306,11 @@ static int vfio_init_device(struct vfio_device *device, struct device *dev,
 			goto out_uninit;
 	}
 
+	/* Allocated before device_initialize() so out_uninit unwind frees it. */
+	ret = device_set_sysfs_lazy(&device->device);
+	if (ret)
+		goto out_uninit;
+
 	device_initialize(&device->device);
 	device->device.release = vfio_device_release;
 	device->device.class = &vfio_device_class;
-- 
2.47.3




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