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

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Message-ID: <20220516183558.GN1343366@nvidia.com>
Date:   Mon, 16 May 2022 15:35:58 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Matthew Rosato <mjrosato@...ux.ibm.com>
Cc:     alex.williamson@...hat.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, 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..

Jason