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

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Message-ID: <20190225142209.GC23257@debian>
Date:   Mon, 25 Feb 2019 15:22:10 +0100
From:   Vincent Stehlé <vincent.stehle@....com>
To:     Eric Auger <eric.auger@...hat.com>
Cc:     eric.auger.pro@...il.com, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        kvmarm@...ts.cs.columbia.edu, joro@...tes.org,
        alex.williamson@...hat.com, jacob.jun.pan@...ux.intel.com,
        yi.l.liu@...ux.intel.com, jean-philippe.brucker@....com,
        will.deacon@....com, robin.murphy@....com, kevin.tian@...el.com,
        ashok.raj@...el.com, marc.zyngier@....com,
        christoffer.dall@....com, peter.maydell@...aro.org
Subject: Re: [PATCH v4 19/22] vfio-pci: Register an iommu fault handler

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@...hat.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.