Date:   Fri, 25 May 2018 14:05:21 -0700
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
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 Fri, 25 May 2018 09:02:23 -0500, Bjorn Helgaas wrote:
> On Thu, May 24, 2018 at 06:20:15PM -0700, Jakub Kicinski wrote:
> > 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.

Think more of an FPGA which can be reprogrammed at runtime to have
different capabilities than an erratum.  Some FWs simply have no use
for VFs and save resources (and validation time) by not supporting it.

> 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.

Okay, perfect.  That makes sense.  The patch below certainly fixes the
first issue for us.  Thank you!

As far as the second issue goes - agreed, having the core reset the
number of VFs to total_VFs definitely makes sense.  It doesn't cater to
the case where FW is reloaded without reprobing, but we don't do this
today anyway.

Should I try to come up with a patch to reset total_VFs after detach?

> 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);
>  

Powered by blists - more mailing lists