Subject: Re: [PATCH 1/2 V3] PCI: introduce device assignment interface and refactory related code

From: Alexander Duyck <[email-masked]>
Date: Mon, 21 Jul 2014 15:41:14 -0700

On 07/21/2014 07:39 AM, Alex Williamson wrote:
> On Mon, 2014-07-21 at 14:11 +0000, Yuval Mintz wrote:
>>>>>>>>>> pci_iov_assign_device(),
>>>>>>>>>> pci_iov_deassign_device()
>>>>>>>>>>
>>>>>>>>>> along with the existed one
>>>>>>>>>>
>>>>>>>>>> pci_vfs_assigned()
>>>>>>>>>>
>>>>>>>>>> They construct the VFs assignment management interface,
>>>>>>> perhaps it's the right place to add some sort of mechanism in
>>>>>>> order to prevent module removal once one of its VFs is assigned.
>>>>>>> [e.g., incrementing module reference count]
>>>>>>>
>>>>>> On what module would the reference count be increased, the PF?
>>>>>> The entire "VF assigned" interface is a hack to work around poor
>>>>>> architectures like legacy KVM device assignment where there's no
>>>>>> proper device owner for the VF.  This is "fixed" by using VFIO
>>>>>> instead and hopefully deprecating legacy KVM assignment.  Thanks,
>>>>>
>>>>>   To explain what Alex said, in another word, if VMs access VF via
>>>>> VFIO driver, the owner of the device is VFIO, in this case, if you
>>>>> unload PF driver, you still need to unload VFIO first, then unload
>>>>> PF driver. but the PF driver knows how to notify the VFIO driver to unload.
>>>>>   But without VFIO like driver, for example some of current code
>>>>> will assign devcie (PF/VF) directly to KVM or XEN without driver in
>>>>> the middle. and the PF driver doesn't know how to unbind the assignment...
>>>>
>>>> I thought about perhaps incrementing the reference count of the PF's
>>>> module [i.e., the module supplying the driver for the PF] directly, so
>>>> THAT module won't be removable as long as the VF is assigned.
>>>>
>>>> Although I don't know whether the module is even accessible; Can you
>>>> derive a pointer to a module from a net_device struct?
>>>
>>> A VF is not necessarily a net device.  A pci_dev does have a physfn pointer
>>> though.
>>
>> I stand corrected.
>>
>> Is there anything inherently wrong about this approach, though?
> 
> What does incrementing the module reference on the PF driver accomplish?
> We can still unbind the PF device from the driver.  That's what this "VF
> used" flag is supposed to accomplish is preventing the PF from unbinding
> from the driver while VFs are in use, but as has been noted, it's a
> terrible hack.
> 

Keep in mind there are also use cases such as what occurs in igb/ixgbe
where the PF driver can be swapped out while the VFs are left in place.
 Nothing explicitly says we cannot unload the PF driver while the VFs
are assigned, we just cannot free the VFs or disable SR-IOV while they
are assigned.

In that scenario we just force the link down on the link state down on
the VFs while the PF is not present.  I'm not sure how many people make
use of the behavior but that is currently how we end up supporting it
for igb/ixgbe.

One approach would be to use a reference count for the SR-IOV itself.  I
know in the case of the igb/ixgbe drivers we use the assigned number of
VFs to determine if we can change the number of VFs allocated or not.  I
imagine you could probably just use the assigned count as a reference
count and if it is non-zero prevent SR-IOV from being disabled.

Thanks,

Alex

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