linux-kernel - Re: [PATCH v4 1/2] vfio/mdev: Check globally for duplicate devices

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Date:   Tue, 22 May 2018 10:13:46 +0200
From:   Cornelia Huck <cohuck@...hat.com>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     kwankhede@...dia.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, pasic@...ux.ibm.com
Subject: Re: [PATCH v4 1/2] vfio/mdev: Check globally for duplicate devices

On Fri, 18 May 2018 13:10:25 -0600
Alex Williamson <alex.williamson@...hat.com> wrote:

> When we create an mdev device, we check for duplicates against the
> parent device and return -EEXIST if found, but the mdev device
> namespace is global since we'll link all devices from the bus.  We do
> catch this later in sysfs_do_create_link_sd() to return -EEXIST, but
> with it comes a kernel warning and stack trace for trying to create
> duplicate sysfs links, which makes it an undesirable response.
> 
> Therefore we should really be looking for duplicates across all mdev
> parent devices, or as implemented here, against our mdev device list.
> Using mdev_list to prevent duplicates means that we can remove
> mdev_parent.lock, but in order not to serialize mdev device creation
> and removal globally, we add mdev_device.active which allows UUIDs to
> be reserved such that we can drop the mdev_list_lock before the mdev
> device is fully in place.
> 
> Two behavioral notes; first, mdev_parent.lock had the side-effect of
> serializing mdev create and remove ops per parent device.  This was
> an implementation detail, not an intentional guarantee provided to
> the mdev vendor drivers.  Vendor drivers can trivially provide this
> serialization internally if necessary.  Second, review comments note
> the new -EAGAIN behavior when the device, and in particular the remove
> attribute, becomes visible in sysfs.  If a remove is triggered prior
> to completion of mdev_device_create() the user will see a -EAGAIN
> error.  While the errno is different, receiving an error during this
> period is not, the previous implementation returned -ENODEV for the
> same condition.  Furthermore, the consistency to the user is improved
> in the case where mdev_device_remove_ops() returns error.  Previously
> concurrent calls to mdev_device_remove() could see the device
> disappear with -ENODEV and return in the case of error.  Now a user
> would see -EAGAIN while the device is in this transitory state.
> 
> Signed-off-by: Alex Williamson <alex.williamson@...hat.com>
> ---
>  Documentation/vfio-mediated-device.txt |    5 ++
>  drivers/vfio/mdev/mdev_core.c          |  102 +++++++++++---------------------
>  drivers/vfio/mdev/mdev_private.h       |    2 -
>  3 files changed, 42 insertions(+), 67 deletions(-)

Reviewed-by: Cornelia Huck <cohuck@...hat.com>

I think it is better to deal with any possible vendor driver
implications on top of this (I still believe that vfio-ccw is fine).