linux-kernel - Re: [PATCH v3 6/6] hisi_acc_vfio_pci: Add support for VFIO live migration

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Message-ID: <20210927160627.GC964074@nvidia.com>
Date:   Mon, 27 Sep 2021 13:06:27 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Leon Romanovsky <leonro@...dia.com>
Cc:     Shameerali Kolothum Thodi <shameerali.kolothum.thodi@...wei.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "mgurtovoy@...dia.com" <mgurtovoy@...dia.com>,
        liulongfang <liulongfang@...wei.com>,
        "Zengtao (B)" <prime.zeng@...ilicon.com>,
        Jonathan Cameron <jonathan.cameron@...wei.com>,
        "Wangzhou (B)" <wangzhou1@...ilicon.com>
Subject: Re: [PATCH v3 6/6] hisi_acc_vfio_pci: Add support for VFIO live
 migration

On Mon, Sep 27, 2021 at 07:00:23PM +0300, Leon Romanovsky wrote:
> On Mon, Sep 27, 2021 at 12:01:19PM -0300, Jason Gunthorpe wrote:
> > On Mon, Sep 27, 2021 at 01:46:31PM +0000, Shameerali Kolothum Thodi wrote:
> > 
> > > > > > Nope, this is locked wrong and has no lifetime management.
> > > > >
> > > > > Ok. Holding the device_lock() sufficient here?
> > > > 
> > > > You can't hold a hisi_qm pointer with some kind of lifecycle
> > > > management of that pointer. device_lock/etc is necessary to call
> > > > pci_get_drvdata()
> > > 
> > > Since this migration driver only supports VF devices and the PF
> > > driver will not be removed until all the VF devices gets removed,
> > > is the locking necessary here?
> > 
> > Oh.. That is really busted up. pci_sriov_disable() is called under the
> > device_lock(pf) and obtains the device_lock(vf).
> 
> Yes, indirectly, but yes.
> 
> > 
> > This means a VF driver can never use the device_lock(pf), otherwise it
> > can deadlock itself if PF removal triggers VF removal.
> 
> VF can use pci_dev_trylock() on PF to prevent PF removal.

no, no here, the device_lock is used in too many places for a trylock
to be appropriate

> > 
> > But you can't access these members without using the device_lock(), as
> > there really are no safety guarentees..
> > 
> > The mlx5 patches have this same sketchy problem.
> > 
> > We may need a new special function 'pci_get_sriov_pf_devdata()' that
> > confirms the vf/pf relationship and explicitly interlocks with the
> > pci_sriov_enable/disable instead of using device_lock()
> > 
> > Leon, what do you think?
> 
> I see pci_dev_lock() and similar functions, they are easier to
> understand that specific pci_get_sriov_pf_devdata().

That is just a wrapper for device_lock - it doesnt help anything

The point is to all out a different locking regime that relies on the
sriov enable/disable removing the VF struct devices

Jason