linux-kernel - Re: [PATCH v7 17/22] vfio-pci/zdev: add open/close device hooks

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Date:   Mon, 16 May 2022 13:38:39 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Matthew Rosato <mjrosato@...ux.ibm.com>,
        linux-s390@...r.kernel.org, cohuck@...hat.com,
        schnelle@...ux.ibm.com, farman@...ux.ibm.com, pmorel@...ux.ibm.com,
        borntraeger@...ux.ibm.com, hca@...ux.ibm.com, gor@...ux.ibm.com,
        gerald.schaefer@...ux.ibm.com, agordeev@...ux.ibm.com,
        svens@...ux.ibm.com, frankja@...ux.ibm.com, david@...hat.com,
        imbrenda@...ux.ibm.com, vneethv@...ux.ibm.com,
        oberpar@...ux.ibm.com, freude@...ux.ibm.com, thuth@...hat.com,
        pasic@...ux.ibm.com, pbonzini@...hat.com, corbet@....net,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH v7 17/22] vfio-pci/zdev: add open/close device hooks

On Mon, 16 May 2022 15:35:58 -0300
Jason Gunthorpe <jgg@...dia.com> wrote:

> On Mon, May 16, 2022 at 02:30:46PM -0400, Matthew Rosato wrote:
> 
> > Conceptually I think this would work for QEMU anyway (it always sets the kvm
> > before we open the device).  I tried to test the idea quickly but couldn't
> > get the following to apply on vfio-next or your vfio_group_locking -- but I
> > understand what you're trying to do so I'll re-work and try it out.  
> 
> I created it on 8c9350e9bf43de1ebab3cc8a80703671e6495ab4 which is the
> vfio_group_locking.. I can send you a github if it helps
> https://github.com/jgunthorpe/linux/commits/vfio_group_lockin
> 
> > @Alex can you think of any usecase/reason why we would need to be able to
> > set the KVM sometime after the device was opened?  Doing something like
> > below would break that, as this introduces the assumption that the group is
> > associated with the KVM before the device is opened (and if it's not, the
> > device open fails).  
> 
> Keep in mind that GVT already hard requires this ordering to even
> allow open_device to work - so it already sets a floor for what
> userspace can do..

How is this going to work when vfio devices are exposed directly?  We
currently have a strict ordering through the group to get to the
device, and it's therefore a reasonable requirement for userspace to
register the group with kvm before opening the device.  Is the notifier
and async KVM registration necessary to support this dependency with
direct device access?  I don't have as clear a picture of the ordering
with with direct access.  Thanks,

Alex