Date:   Fri, 25 May 2018 15:27:52 -0400
From:   Don Dutile <ddutile@...hat.com>
To:     Bjorn Helgaas <helgaas@...nel.org>,
        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>,
        oss-drivers@...ronome.com, Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH] PCI: allow drivers to limit the number of VFs to 0

On 05/25/2018 10:02 AM, 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.
> 
+1.

> But the current implementation does not allow a PF driver to limit VFs
> to 0, and that does seem nonsensical.
> 
Well, not really -- claiming to support VFs, and then wanting it to be 0...
I could certainly argue is non-sensical.
 From a sw perspective, sure, see if we can set VFs to 0 (and reset to another value later).

/me wishes that implementers would follow the architecture vs torquing it into strange shapes.

>> 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
um, if it's at sriov_init() how is max changed by a PF driver?
or am I missing something subtle (a new sysfs param) as to what is being changed?

> 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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