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

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Date:   Mon, 16 Dec 2019 08:46:45 +0000
From:   Parav Pandit <parav@...lanox.com>
To:     Shannon Nelson <snelson@...sando.io>,
        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 12/16/2019 11:32 AM, Shannon Nelson wrote:
> On 12/15/19 8:47 PM, Parav Pandit wrote:
>> On 12/13/2019 4:10 AM, Shannon Nelson wrote:
>>> 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.
>> Please do not cache num_vfs in driver. Use the pci core's pci_num_vf()
>> in the new code that you are adding.
> 
> I disagree.  The pci_num_vf() tells us what the kernel has set up for
> VFs running, while the driver's num_vfs tracks how many resources the
> driver has set up for handling VFs: these are two different numbers, and
> there are times in the life of the driver when these numbers are
> different.  Yes, these are small windows of time, but they are different
> and need to be treated differently.
> 
They shouldn't be different. Why are they different?

>> More below.
>>> 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
>>>
>> I not have strong opinion on letting sriov enabled/disabled on PF device
>> removal.
>> But it should be symmetric on probe() and remove() for PF.
>> If you want to keep it enabled on PF removal, you need to check on probe
>> and allocate VF metadata you have by using helper function in
>> sriov_configure() and in probe().
>> This is followed by mlx5 driver.
> 
> Agreed, and this check at probe time is included in the v3 patch that I
> sent out on Friday.
> 
ok. Will review. Thanks.

> sln
> 
>