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Date:   Fri, 6 Mar 2020 08:50:05 -0700
From:   Alex Williamson <alex.williamson@redhat.com>
To:     "Tian, Kevin" <kevin.tian@intel.com>
Cc:     "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "linux-pci@vger.kernel.org" <linux-pci@vger.kernel.org>,
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        "mtosatti@redhat.com" <mtosatti@redhat.com>,
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        "Richardson, Bruce" <bruce.richardson@intel.com>,
        "cohuck@redhat.com" <cohuck@redhat.com>
Subject: Re: [PATCH v2 5/7] vfio/pci: Add sriov_configure support
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On Fri, 6 Mar 2020 09:45:40 +0000
"Tian, Kevin" <kevin.tian@intel.com> wrote:

> > From: Tian, Kevin
> > Sent: Friday, March 6, 2020 3:57 PM
> >   
> > > From: Alex Williamson <alex.williamson@redhat.com>
> > > Sent: Friday, March 6, 2020 2:23 AM
> > >
> > > On Tue, 25 Feb 2020 03:08:00 +0000
> > > "Tian, Kevin" <kevin.tian@intel.com> wrote:
> > >  
> > > > > From: Alex Williamson
> > > > > Sent: Thursday, February 20, 2020 2:54 AM
> > > > >
> > > > > With the VF Token interface we can now expect that a vfio userspace
> > > > > driver must be in collaboration with the PF driver, an unwitting
> > > > > userspace driver will not be able to get past the GET_DEVICE_FD step
> > > > > in accessing the device.  We can now move on to actually allowing
> > > > > SR-IOV to be enabled by vfio-pci on the PF.  Support for this is not
> > > > > enabled by default in this commit, but it does provide a module option
> > > > > for this to be enabled (enable_sriov=1).  Enabling VFs is rather
> > > > > straightforward, except we don't want to risk that a VF might get
> > > > > autoprobed and bound to other drivers, so a bus notifier is used to
> > > > > "capture" VFs to vfio-pci using the driver_override support.  We
> > > > > assume any later action to bind the device to other drivers is
> > > > > condoned by the system admin and allow it with a log warning.
> > > > >
> > > > > vfio-pci will disable SR-IOV on a PF before releasing the device,
> > > > > allowing a VF driver to be assured other drivers cannot take over the
> > > > > PF and that any other userspace driver must know the shared VF token.
> > > > > This support also does not provide a mechanism for the PF userspace
> > > > > driver itself to manipulate SR-IOV through the vfio API.  With this
> > > > > patch SR-IOV can only be enabled via the host sysfs interface and the
> > > > > PF driver user cannot create or remove VFs.  
> > > >
> > > > I'm not sure how many devices can be properly configured simply
> > > > with pci_enable_sriov. It is not unusual to require PF driver prepare
> > > > something before turning PCI SR-IOV capability. If you look kernel
> > > > PF drivers, there are only two using generic pci_sriov_configure_
> > > > simple (simple wrapper like pci_enable_sriov), while most others
> > > > implementing their own callback. However vfio itself has no idea
> > > > thus I'm not sure how an user knows whether using this option can
> > > > actually meet his purpose. I may miss something here, possibly
> > > > using DPDK as an example will make it clearer.  
> > >
> > > There is still the entire vfio userspace driver interface.  Imagine for
> > > example that QEMU emulates the SR-IOV capability and makes a call out
> > > to libvirt (or maybe runs with privs for the PF SR-IOV sysfs attribs)
> > > when the guest enables SR-IOV.  Can't we assume that any PF specific
> > > support can still be performed in the userspace/guest driver, leaving
> > > us with a very simple and generic sriov_configure callback in vfio-pci?  
> > 
> > Makes sense. One concern, though, is how an user could be warned
> > if he inadvertently uses sysfs to enable SR-IOV on a vfio device whose
> > userspace driver is incapable of handling it. Note any VFIO device,
> > if SR-IOV capable, will allow user to do so once the module option is
> > turned on and the callback is registered. I felt such uncertainty can be
> > contained by toggling SR-IOV through a vfio api, but from your description
> > obviously it is what you want to avoid. Is it due to the sequence reason,
> > e.g. that SR-IOV must be enabled before userspace PF driver sets the
> > token?
> >   
> 
> reading again I found that you specifically mentioned "the PF driver user 
> cannot create or remove VFs.". However I failed to get the rationale 
> behind. If the VF drivers have built the trust with the PF driver through
> the token, what is the problem of allowing the PF driver to further manage 
> SR-IOV itself? suppose any VF removal will be done in a cooperate way
> to avoid surprise impact to related VF drivers. then possibly a new vfio
> ioctl for setting the VF numbers plus a token from the userspace driver
> could also serve the purpose of this patch series (GET_DEVICE_FD + sysfs)?

If a user is allowed to create VFs, does that user automatically get
ownership of those devices?  How is that accomplished?  What if we want
to make use of the VF via a separate process?  How do we coordinate
that with the PF driver?  All of these problems are resolved if we
assume the userspace PF driver needs to operate in collaboration with a
privileged entity to interact with sysfs to configure SR-IOV and manage
the resulting VFs.  I have no desire to take on that responsibility
within vfio-pci and I also feel that a user owning a PF device should
not inherently grant that user the ability to create and remove other
devices on the host, even if they are sourced from the PF.  Thanks,

Alex