linux-kernel - Re: [RFC PATCH 1/1] vfio-pci/iommu: Detach iommu group on remove path

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Date:	Wed, 22 Jul 2015 10:54:35 -0600
From:	Alex Williamson <alex.williamson@...hat.com>
To:	Gerald Schaefer <gerald.schaefer@...ibm.com>
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Sebastian Ott <sebott@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH 1/1] vfio-pci/iommu: Detach iommu group on remove
 path

On Tue, 2015-07-21 at 19:44 +0200, Gerald Schaefer wrote:
> When a user completes the VFIO_SET_IOMMU ioctl and the vfio-pci device is
> removed thereafter (before any other ioctl like VFIO_GROUP_GET_DEVICE_FD),
> then the detach_dev callback of the underlying IOMMU API is never called.
> 
> This patch adds a call to vfio_group_try_dissolve_container() to the remove
> path, which will trigger the missing detach_dev callback in this scenario.
> 
> Signed-off-by: Gerald Schaefer <gerald.schaefer@...ibm.com>
> ---
>  drivers/vfio/vfio.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c
> index 2fb29df..9c5c784 100644
> --- a/drivers/vfio/vfio.c
> +++ b/drivers/vfio/vfio.c
> @@ -711,6 +711,8 @@ static bool vfio_dev_present(struct vfio_group *group, struct device *dev)
>  	return true;
>  }
>  
> +static void vfio_group_try_dissolve_container(struct vfio_group *group);
> +
>  /*
>   * Decrement the device reference count and wait for the device to be
>   * removed.  Open file descriptors for the device... */
> @@ -785,6 +787,7 @@ void *vfio_del_group_dev(struct device *dev)
>  		}
>  	} while (ret <= 0);
>  
> +	vfio_group_try_dissolve_container(group);
>  	vfio_group_put(group);
>  
>  	return device_data;

This won't work, vfio_group_try_dissolve_container() decrements
container_users, which an unused device is not.  Imagine if we had more
than one device in the iommu group, one device is removed and the
container is dissolved despite the user holding a reference and other
viable devices remaining.  Additionally, from an isolation perspective,
an unbind from vfio-pci should not pull the device out of the iommu
domain, it's part of the domain because it's not isolated and that
continues even after unbind.

I think what you want to do is detach a device from the iommu domain
only when it's being removed from iommu group, such as through
iommu_group_remove_device().  We already have a bit of an asymmetry
there as iommu_group_add_device() will add devices to the currently
active iommu domain for the group, but iommu_group_remove_device() does
not appear to do the reverse.  Thanks,

Alex

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