Re: [PATCH] PCI: allow drivers to limit the number of VFs to 0

[Bjorn Helgaas <helgaas@kernel.org>]

[+cc liquidio, benet, fm10k maintainers:

  The patch below will affect you if your driver calls
    pci_sriov_set_totalvfs(dev, 0);

  Previously that caused a subsequent pci_sriov_get_totalvfs() to return
  the totalVFs value from the SR-IOV capability.  After this patch, it will
  return 0, which has implications for VF enablement via the sysfs
  "sriov_numvfs" file.]

On Fri, May 25, 2018 at 09:02:23AM -0500, Bjorn Helgaas wrote:
> On Thu, May 24, 2018 at 06:20:15PM -0700, Jakub Kicinski wrote:
> > Hi Bjorn!
> > 
> > On Thu, 24 May 2018 18:57:48 -0500, Bjorn Helgaas wrote:
> > > On Mon, Apr 02, 2018 at 03:46:52PM -0700, Jakub Kicinski wrote:
> > > > Some user space depends on enabling sriov_totalvfs number of VFs
> > > > to not fail, e.g.:
> > > > 
> > > > $ cat .../sriov_totalvfs > .../sriov_numvfs
> > > > 
> > > > For devices which VF support depends on loaded FW we have the
> > > > pci_sriov_{g,s}et_totalvfs() API.  However, this API uses 0 as
> > > > a special "unset" value, meaning drivers can't limit sriov_totalvfs
> > > > to 0.  Remove the special values completely and simply initialize
> > > > driver_max_VFs to total_VFs.  Then always use driver_max_VFs.
> > > > Add a helper for drivers to reset the VF limit back to total.  
> > > 
> > > I still can't really make sense out of the changelog.
> > >
> > > I think part of the reason it's confusing is because there are two
> > > things going on:
> > > 
> > >   1) You want this:
> > >   
> > >        pci_sriov_set_totalvfs(dev, 0);
> > >        x = pci_sriov_get_totalvfs(dev) 
> > > 
> > >      to return 0 instead of total_VFs.  That seems to connect with
> > >      your subject line.  It means "sriov_totalvfs" in sysfs could be
> > >      0, but I don't know how that is useful (I'm sure it is; just
> > >      educate me :))
> > 
> > Let me just quote the bug report that got filed on our internal bug
> > tracker :)
> > 
> >   When testing Juju Openstack with Ubuntu 18.04, enabling SR-IOV causes
> >   errors because Juju gets the sriov_totalvfs for SR-IOV-capable device
> >   then tries to set that as the sriov_numvfs parameter.
> > 
> >   For SR-IOV incapable FW, the sriov_totalvfs parameter should be 0, 
> >   but it's set to max.  When FW is switched to flower*, the correct 
> >   sriov_totalvfs value is presented.
> > 
> > * flower is a project name
> 
> From the point of view of the PCI core (which knows nothing about
> device firmware and relies on the architected config space described
> by the PCIe spec), this sounds like an erratum: with some firmware
> installed, the device is not capable of SR-IOV, but still advertises
> an SR-IOV capability with "TotalVFs > 0".
> 
> Regardless of whether that's an erratum, we do allow PF drivers to use
> pci_sriov_set_totalvfs() to limit the number of VFs that may be
> enabled by writing to the PF's "sriov_numvfs" sysfs file.
> 
> But the current implementation does not allow a PF driver to limit VFs
> to 0, and that does seem nonsensical.
> 
> > My understanding is OpenStack uses sriov_totalvfs to determine how many
> > VFs can be enabled, looks like this is the code:
> > 
> > http://git.openstack.org/cgit/openstack/charm-neutron-openvswitch/tree/hooks/neutron_ovs_utils.py#n464
> > 
> > >   2) You're adding the pci_sriov_reset_totalvfs() interface.  I'm not
> > >      sure what you intend for this.  Is *every* driver supposed to
> > >      call it in .remove()?  Could/should this be done in the core
> > >      somehow instead of depending on every driver?
> > 
> > Good question, I was just thinking yesterday we may want to call it
> > from the core, but I don't think it's strictly necessary nor always
> > sufficient (we may reload FW without re-probing).
> > 
> > We have a device which supports different number of VFs based on the FW
> > loaded.  Some legacy FWs does not inform the driver how many VFs it can
> > support, because it supports max.  So the flow in our driver is this:
> > 
> > load_fw(dev);
> > ...
> > max_vfs = ask_fw_for_max_vfs(dev);
> > if (max_vfs >= 0)
> > 	return pci_sriov_set_totalvfs(dev, max_vfs);
> > else /* FW didn't tell us, assume max */
> > 	return pci_sriov_reset_totalvfs(dev); 
> > 
> > We also reset the max on device remove, but that's not strictly
> > necessary.
> > 
> > Other users of pci_sriov_set_totalvfs() always know the value to set
> > the total to (either always get it from FW or it's a constant).
> > 
> > If you prefer we can work out the correct max for those legacy cases in
> > the driver as well, although it seemed cleaner to just ask the core,
> > since it already has total_VFs value handy :)
> > 
> > > I'm also having a hard time connecting your user-space command example
> > > with the rest of this.  Maybe it will make more sense to me tomorrow
> > > after some coffee.
> > 
> > OpenStack assumes it will always be able to set sriov_numvfs to
> > sriov_totalvfs, see this 'if':
> > 
> > http://git.openstack.org/cgit/openstack/charm-neutron-openvswitch/tree/hooks/neutron_ovs_utils.py#n512
> 
> Thanks for educating me.  I think there are two issues here that we
> can separate.  I extracted the patch below for the first.
> 
> The second is the question of resetting driver_max_VFs.  I think we
> currently have a general issue in the core:
> 
>   - load PF driver 1
>   - driver calls pci_sriov_set_totalvfs() to reduce driver_max_VFs
>   - unload PF driver 1
>   - load PF driver 2
> 
> Now driver_max_VFs is still stuck at the lower value set by driver 1.
> I don't think that's the way this should work.
> 
> I guess this is partly a consequence of setting driver_max_VFs in
> sriov_init(), which is called before driver attach and should only
> depend on hardware characteristics, so it is related to the patch
> below.  But I think we should fix it in general, not just for
> netronome.
> 
> 
> commit 4a338bc6f94b9ad824ac944f5dfc249d6838719c
> Author: Jakub Kicinski <jakub.kicinski@netronome.com>
> Date:   Fri May 25 08:18:34 2018 -0500
> 
>     PCI/IOV: Allow PF drivers to limit total_VFs to 0
>     
>     Some SR-IOV PF drivers implement .sriov_configure(), which allows
>     user-space to enable VFs by writing the desired number of VFs to the sysfs
>     "sriov_numvfs" file (see sriov_numvfs_store()).
>     
>     The PCI core limits the number of VFs to the TotalVFs advertised by the
>     device in its SR-IOV capability.  The PF driver can limit the number of VFs
>     to even fewer (it may have pre-allocated data structures or knowledge of
>     device limitations) by calling pci_sriov_set_totalvfs(), but previously it
>     could not limit the VFs to 0.
>     
>     Change pci_sriov_get_totalvfs() so it always respects the VF limit imposed
>     by the PF driver, even if the limit is 0.
>     
>     This sequence:
>     
>       pci_sriov_set_totalvfs(dev, 0);
>       x = pci_sriov_get_totalvfs(dev);
>     
>     previously set "x" to TotalVFs from the SR-IOV capability.  Now it will set
>     "x" to 0.
>     
>     Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
>     Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
> 
> diff --git a/drivers/pci/iov.c b/drivers/pci/iov.c
> index 192b82898a38..d0d73dbbd5ca 100644
> --- a/drivers/pci/iov.c
> +++ b/drivers/pci/iov.c
> @@ -469,6 +469,7 @@ static int sriov_init(struct pci_dev *dev, int pos)
>  	iov->nres = nres;
>  	iov->ctrl = ctrl;
>  	iov->total_VFs = total;
> +	iov->driver_max_VFs = total;
>  	pci_read_config_word(dev, pos + PCI_SRIOV_VF_DID, &iov->vf_device);
>  	iov->pgsz = pgsz;
>  	iov->self = dev;
> @@ -827,10 +828,7 @@ int pci_sriov_get_totalvfs(struct pci_dev *dev)
>  	if (!dev->is_physfn)
>  		return 0;
>  
> -	if (dev->sriov->driver_max_VFs)
> -		return dev->sriov->driver_max_VFs;
> -
> -	return dev->sriov->total_VFs;
> +	return dev->sriov->driver_max_VFs;
>  }
>  EXPORT_SYMBOL_GPL(pci_sriov_get_totalvfs);
>