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Date:   Sun, 8 Mar 2020 18:46:10 -0600
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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        "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 Sat, 7 Mar 2020 01:35:23 +0000
"Tian, Kevin" <kevin.tian@intel.com> wrote:

> > From: Alex Williamson
> > Sent: Saturday, March 7, 2020 6:18 AM
> > 
> > On Fri, 6 Mar 2020 07:57:19 +0000
> > "Tian, Kevin" <kevin.tian@intel.com> wrote:
> >   
> > > > 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?  
> > 
> > As in my other reply, enabling SR-IOV via a vfio API suggests that
> > we're not only granting the user owning the PF device access to the
> > device itself, but also the ability to create and remove subordinate
> > devices on the host.  That implies an extended degree of trust in the
> > user beyond the PF device itself and raises questions about whether a
> > user who is allowed to create VF devices should automatically be
> > granted access to those VF devices, what the mechanism would be for
> > that, and how we might re-assign those devices to other users,
> > potentially including host kernel usage.  What I'm proposing here
> > doesn't preclude some future extension in that direction, but instead
> > tries to simplify a first step towards enabling SR-IOV by leaving the
> > SR-IOV enablement and VF assignment in the realm of a privileged system
> > entity.  
> 
> the intention is clear to me now.
> 
> > 
> > So, what I think you're suggesting here is that we should restrict
> > vfio_pci_sriov_configure() to reject enabling SR-IOV until a user
> > driver has configured a VF token.  That requires both that the
> > userspace driver has initialized to this point before SR-IOV can be
> > enabled and that we would be forced to define a termination point for
> > the user set VF token.  Logically, this would need to be when the
> > userspace driver exits or closes the PF device, which implies that we
> > need to disable SR-IOV on the PF at this point, or we're left in an
> > inconsistent state where VFs are enabled but cannot be disabled because
> > we don't have a valid VF token.  Now we're back to nearly a state where
> > the user has control of not creating devices on the host, but removing
> > them by closing the device, which will necessarily require that any VF
> > driver release the device, whether userspace or kernel.
> > 
> > I'm not sure what we're gaining by doing this though.  I agree that
> > there will be users that enable SR-IOV on a PF and then try to, for
> > example, assign the PF and all the VFs to a VM.  The VFs will fail due
> > to lacking VF token support, unless they've patch QEMU with my test
> > code, but depending on the PF driver in the guest, it may, or more
> > likely won't work.  But don't you think the VFs and probably PF not
> > working is a sufficient clue that the configuration is invalid?  OTOH,
> > from what I've heard of the device in the ID table of the pci-pf-stub
> > driver, they might very well be able to work with both PF and VFs in
> > QEMU using only my test code to set the VF token.
> > 
> > Therefore, I'm afraid what you're asking for here is to impose a usage
> > restriction as a sanity test, when we don't really know what might be
> > sane for this particular piece of hardware or use case.  There are
> > infinite ways that a vfio based userspace driver can fail to configure
> > their hardware and make it work correctly, many of them are device
> > specific.  Isn't this just one of those cases?  Thanks,
> >   
> 
> what you said all makes sense. so I withdraw the idea of manipulating
> SR-IOV through vfio ioctl. However I still feel that simply registering 
> sriov_configuration callback by vfio-pci somehow violates the typical
> expectation of the sysfs interface. Before this patch, the success return
> of writing non-zero value to numvfs implies VFs are in sane state and
> functionally ready for immediate use. However now the behavior of
> success return becomes undefined for vfio devices, since even vfio-pci 
> itself doesn't know whether VFs are functional for a random device 
> (may know some if carrying the same device IDs from pci-pf-stub). It
> simply relies on the privileged entity who knows exactly the implication
> of such write, while there is no way to warn inadvertent users which
> to me is not a good design from kernel API p.o.v. Of course we may 
> document such restriction and the driver_override may also be an 
> indirect way to warn such user if he wants to use VFs for other purpose.
> But it is still less elegant than reporting it in the first place. Maybe
> what we really require is a new sysfs attribute purely for enabling
> PCI SR-IOV capability, which doesn't imply making VFs actually 
> functional as did through the existing numvfs?

I don't read the same guarantee into the sysfs SR-IOV interface.  If
such a guarantee exists, it's already broken by pci-pf-stub, which like
vfio-pci allows dynamic IDs and driver_override to bind to any PF device
allowing the ability to create (potentially) non-functional VFs.  I
think it would be a really bad decision to fork a new sysfs interface
for this.  I've already made SR-IOV support in vfio-pci an opt-in via a
module option, would it ease your concerns if I elaborate in the text
for the option that enabling SR-IOV may depend on support provided by a
vfio-pci userspace driver?

I think that without absolutely knowing that an operation is incorrect,
we're just generating noise and confusion by triggering warnings or
developing alternate interfaces.  Unfortunately, we have no generic
means of knowing that an operation is incorrect, so I assume the best.
Thanks,

Alex