Re: [PATCH v4 19/22] vfio-pci: Register an iommu fault handler

Vincent Stehlé
2019-02-25T14:22:10Z

Hi Eric,

On Mon, Feb 18, 2019 at 02:55:00PM +0100, Eric Auger wrote:
> This patch registers a fault handler which records faults in
> a circular buffer and then signals an eventfd. This buffer is
> exposed within the fault region.
>
> Signed-off-by: Eric Auger <eric.auger@redhat.com>
> ---
> drivers/vfio/pci/vfio_pci.c | 49 +++++++++++++++++++++++++++++
> drivers/vfio/pci/vfio_pci_private.h | 1 +
> 2 files changed, 50 insertions(+)
>
> diff --git a/drivers/vfio/pci/vfio_pci.c b/drivers/vfio/pci/vfio_pci.c
> index aaf63e5ca2b6..019c9fd380a5 100644
> --- a/drivers/vfio/pci/vfio_pci.c
> +++ b/drivers/vfio/pci/vfio_pci.c
(..)
> static int vfio_pci_init_fault_region(struct vfio_pci_device *vdev)
> {
> struct vfio_region_fault_prod *header;
> @@ -276,6 +317,13 @@ static int vfio_pci_init_fault_region(struct vfio_pci_device *vdev)
> header = (struct vfio_region_fault_prod *)vdev->fault_pages;
> header->version = -1;
> header->offset = PAGE_SIZE;
> +
> + ret = iommu_register_device_fault_handler(&vdev->pdev->dev,
> + vfio_pci_iommu_dev_fault_handler,
> + vdev);
> + if (ret)
> + goto out;
> +
> return 0;
> out:
> kfree(vdev->fault_pages);

This patch calls iommu_register_device_fault_handler from
vfio_pci_init_fault_region, leading to the following call stack:

iommu_register_device_fault_handler
vfio_pci_init_fault_region
vfio_pci_enable
vfio_pci_open
vfio_group_get_device_fd

> @@ -1420,6 +1468,7 @@ static void vfio_pci_remove(struct pci_dev *pdev)
> vfio_iommu_group_put(pdev->dev.iommu_group, &pdev->dev);
> kfree(vdev->region);
> kfree(vdev->fault_pages);
> + iommu_unregister_device_fault_handler(&pdev->dev);
> mutex_destroy(&vdev->ioeventfds_lock);
> kfree(vdev);

And then this patch calls iommu_unregister_device_fault_handler from
vfio_pci_remove, and not from vfio_pci_release.

I think this means a device cannot be used twice in a row without unloading the
module.

Here is an example sequence:

1. modprobe vfio-pci
2. Userspace uses VFIO, calls ioctl(VFIO_GROUP_GET_DEVICE_FD)
2.1. iommu_register_device_fault_handler is called
3. Userspace exits
3.1. vfio_pci_release is called,
but iommu_unregister_device_fault_handler is not called
4. Userspace uses VFIO agin, calls ioctl(VFIO_GROUP_GET_DEVICE_FD) again
4.1. iommu_register_device_fault_handler is called again,
notices a fault handler is already there,
returns -EBUSY

Unloading the vfio-pci module will call vfio_pci_remove.

Maybe iommu_unregister_device_fault_handler should be called from
vfio_pci_release instead of vfio_pci_remove?

Best regards,
Vincent.