linux-kernel - Re: [PATCH v3 1/1] s390/vfio-ap: fix circular lockdep when setting/clearing crypto masks

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Date:   Wed, 3 Mar 2021 20:42:11 +0100
From:   Halil Pasic <pasic@...ux.ibm.com>
To:     Tony Krowiak <akrowiak@...ux.ibm.com>
Cc:     linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, stable@...r.kernel.org,
        borntraeger@...ibm.com, cohuck@...hat.com, kwankhede@...dia.com,
        pbonzini@...hat.com, alex.williamson@...hat.com,
        pasic@...ux.vnet.ibm.com
Subject: Re: [PATCH v3 1/1] s390/vfio-ap: fix circular lockdep when
 setting/clearing crypto masks

On Wed, 3 Mar 2021 11:41:22 -0500
Tony Krowiak <akrowiak@...ux.ibm.com> wrote:

> > How do you exect userspace to react to this -ENODEV?  
> 
> The VFIO_DEVICE_RESET ioctl expects a return code.
> The vfio_ap_mdev_reset_queues() function can return -EIO or
> -EBUSY, so I would expect userspace to handle -ENODEV
> similarly to -EIO or any other non-zero return code. I also
> looked at all of the VFIO_DEVICE_RESET calls from QEMU to see
> how the return from the ioctl call is handled:
> 
> * ap: reports the reset failed along with the rc

And carries on as if nothing happened. There is not much smart
userspace can do in such a situation. Therefore the reset really
should not fail.

Please note that in this particular case, if the userspace would
opt for a retry, we would most likely end up in a retry loop.

> * ccw: doesn't check the rc
> * pci: kind of hard to follow without digging deep, but definitely
>           handles non-zero rc.
> 
> I think the caller should be notified whether the queues were
> successfully reset or not, and why; in this case, the answer is
> there are no devices to reset.

That is the wrong answer. The ioctl is supposed to reset the
ap_matrix_mdev device. The ap_matrix_mdev device still exists. Thus
returning -ENODEV is bugous.

Regards,
Halil