Date:   Fri, 25 May 2018 09:02:23 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Jakub Kicinski <jakub.kicinski@...ronome.com>
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org,
        netdev@...r.kernel.org, Sathya Perla <sathya.perla@...adcom.com>,
        Felix Manlunas <felix.manlunas@...iumnetworks.com>,
        alexander.duyck@...il.com, john.fastabend@...il.com,
        Jacob Keller <jacob.e.keller@...el.com>,
        Donald Dutile <ddutile@...hat.com>, oss-drivers@...ronome.com,
        Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH] PCI: allow drivers to limit the number of VFs to 0

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@...ronome.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@...ronome.com>
    Signed-off-by: Bjorn Helgaas <bhelgaas@...gle.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);
 

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