netdev - Re: [PATCH v2 net-next 2/2] ionic: support sr-iov operations

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Date:   Thu, 12 Dec 2019 14:40:01 -0800
From:   Shannon Nelson <snelson@...sando.io>
To:     Parav Pandit <parav@...lanox.com>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "davem@...emloft.net" <davem@...emloft.net>
Subject: Re: [PATCH v2 net-next 2/2] ionic: support sr-iov operations

On 12/12/19 2:24 PM, Parav Pandit wrote:
> On 12/12/2019 3:35 PM, Jakub Kicinski wrote:
>> On Thu, 12 Dec 2019 11:59:50 -0800, Shannon Nelson wrote:
>>> On 12/12/19 11:52 AM, Jakub Kicinski wrote:
>>>> On Thu, 12 Dec 2019 06:53:42 +0000, Parav Pandit wrote:
>>>>>>    static void ionic_remove(struct pci_dev *pdev)
>>>>>>    {
>>>>>>    	struct ionic *ionic = pci_get_drvdata(pdev);
>>>>>> @@ -257,6 +338,9 @@ static void ionic_remove(struct pci_dev *pdev)
>>>>>>    	if (!ionic)
>>>>>>    		return;
>>>>>>    
>>>>>> +	if (pci_num_vf(pdev))
>>>>>> +		ionic_sriov_configure(pdev, 0);
>>>>>> +
>>>>> Usually sriov is left enabled while removing PF.
>>>>> It is not the role of the pci PF removal to disable it sriov.
>>>> I don't think that's true. I consider igb and ixgbe to set the standard
>>>> for legacy SR-IOV handling since they were one of the first (the first?)
>>>> and Alex Duyck wrote them.
>>>>
>>>> mlx4, bnxt and nfp all disable SR-IOV on remove.
>>> This was my understanding as well, but now I can see that ixgbe and i40e
>>> are both checking for existing VFs in probe and setting up to use them,
>>> as well as the newer ice driver.  I found this today by looking for
>>> where they use pci_num_vf().
>> Right, if the VFs very already enabled on probe they are set up.
>>
>> It's a bit of a asymmetric design, in case some other driver left
>> SR-IOV on, I guess.
>>
> I remember on one email thread on netdev list from someone that in one
> use case, they upgrade the PF driver while VFs are still bound and
> SR-IOV kept enabled.
> I am not sure how much it is used in practice/or practical.
> Such use case may be the reason to keep SR-IOV enabled.

This brings up a potential corner case where it would be better for the 
driver to use its own num_vfs value rather than relying on the 
pci_num_vf() when answering the ndo_get_vf_*() callbacks, and at least 
the igb may be susceptible.  If the driver hasn't set up its vf[] data 
arrays because there was an error in setting them up in the probe(), and 
later someone tries to get VF statistics, the ndo_get_vf_stats callback 
could end up dereferencing bad pointers because vf is less than 
pci_num_vf() but more than the number of vf[] structs set up by the driver.

I suppose the argument could be made that PF's probe should if the VF 
config fails, but it might be nice to have the PF driver running to help 
fix up whatever when sideways in the VF configuration.

sln