linux-kernel - Re: [PATCH v3 1/9] s390: vfio_ap: link the vfio_ap devices to the vfio

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Date:   Thu, 14 Feb 2019 16:40:44 +0100
From:   Cornelia Huck <cohuck@...hat.com>
To:     Christian Borntraeger <borntraeger@...ibm.com>
Cc:     Pierre Morel <pmorel@...ux.ibm.com>, alex.williamson@...hat.com,
        linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
        kvm@...r.kernel.org, frankja@...ux.ibm.com, akrowiak@...ux.ibm.com,
        pasic@...ux.ibm.com, david@...hat.com, schwidefsky@...ibm.com,
        heiko.carstens@...ibm.com, freude@...ux.ibm.com, mimu@...ux.ibm.com
Subject: Re: [PATCH v3 1/9] s390: vfio_ap: link the vfio_ap devices to the
 vfio_ap bus subsystem

On Thu, 14 Feb 2019 16:05:35 +0100
Christian Borntraeger <borntraeger@...ibm.com> wrote:

> On 14.02.2019 15:54, Cornelia Huck wrote:
> > On Thu, 14 Feb 2019 14:51:01 +0100
> > Pierre Morel <pmorel@...ux.ibm.com> wrote:  
> 
> Pierre,
> this is independent from this series and should have been sent separately.
> In the end (when we have the final solution) this will require cc stable.
> >   
> >> Libudev relies on having a subsystem link for non-root devices. To
> >> avoid libudev (and potentially other userspace tools) choking on the
> >> matrix device let us introduce a vfio_ap bus and with that the vfio_ap
> >> bus subsytem, and make the matrix device reside within it.  
> > 
> > How does libudev choke on this? It feels wrong to introduce a bus that
> > basically does nothing...  
> 
> I have seen libvirt looping when a matrix device was available before the
> libvirt start.
> Marc Hartmayer debugged this and circumvented this in libvirt:
> https://www.redhat.com/archives/libvir-list/2019-February/msg00837.html
> 
> Still libudev expects a subsystem link in the matrix folder when doing the 
> udev_enumerate_scan_devices call.
> 
> Having a bus is one way of adding a subsystem link.

Ugh, that feels really broken. Adding a bus is probably the easiest way
to get this working, though :/