linux-kernel - Re: [PATCH] pci/sriov: Add an option to probe VFs or not before enabling SR-IOV

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Date:   Mon, 20 Mar 2017 22:38:43 -0500
From:   Bodong Wang <bodong@...lanox.com>
To:     Gavin Shan <gwshan@...ux.vnet.ibm.com>
CC:     <bhelgaas@...gle.com>, <linux-pci@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <saeedm@...lanox.com>,
        Eli Cohen <eli@...lanox.com>
Subject: Re: [PATCH] pci/sriov: Add an option to probe VFs or not before
 enabling SR-IOV

On 3/20/2017 7:24 PM, Gavin Shan wrote:
> On Mon, Mar 20, 2017 at 06:34:23PM -0500, Bodong Wang wrote:
>> On 3/20/2017 6:07 PM, Gavin Shan wrote:
>>> On Mon, Mar 20, 2017 at 05:14:34PM +0200, bodong@...lanox.com wrote:
>>>> From: Bodong Wang <bodong@...lanox.com>
>>>>
>>>> Sometimes it is not desirable to probe the virtual functions after
>>>> SRIOV is enabled. This can save host side resource usage by VF
>>>> instances which would be eventually probed to VMs.
>>>>
>>>> Added a new PCI sysfs interface "sriov_probe_vfs" to control that
>>> >from PF, all current callers still retain the same functionality.
>>>> To modify it, echo 0/n/N (disable probe) or 1/y/Y (enable probe) to
>>>>
>>>> /sys/bus/pci/devices/<DOMAIN:BUS:DEVICE.FUNCTION>/sriov_probe_vfs
>>>>
>>>> Note that, the choice must be made before enabling VFs. The change
>>>> will not take effect if VFs are already enabled. Simply, one can set
>>>> sriov_numvfs to 0, choose whether to probe or not, and then resume
>>>> sriov_numvfs.
>>>>
>>> Bodong, I'm not sure if there is a requirement to load driver for the
>>> specified number of VFs? That indicates no driver will be loaded for
>>> other VFs. If so, this interface might serve the purpose as well.
>> Gavin, thanks for the review. That is indeed an interesting suggestion.
>> Theoretically,  we can change that probe_vfs from boolean to integer. And use
>> it as a counter to probe the first N VFs(if N < total_vfs).  Let's see if
>> there are any objections.
> Ok.
>
>>> +#ifdef CONFIG_PCI_IOV
>>> +	if (!pci_dev->is_virtfn ||
>>> +	    (pci_dev->is_virtfn && pci_dev->physfn->sriov->probe_vfs)) {
>>> +#endif
>>> +		error = __pci_device_probe(drv, pci_dev);
>>> +		if (error) {
>>> +			pcibios_free_irq(pci_dev);
>>> +			pci_dev_put(pci_dev);
>>> +		}
>>> +#ifdef CONFIG_PCI_IOV
>>> 	}
>>> +#endif
>>>
>>> I think it's reasonable to have a inline function for this check:
>> It's doable, but what's the benefit?
>>> #ifdef CONFIG_PCI_IOV
>>> static inline bool pci_device_can_probe(struct pci_dev *pdev)
>>> {
>>> 	return (!pdev->is_virtfn || pdev->physfn->sriov->probe_vfs);
>> should be return (!pdev->is_virtfn || (pci_dev->is_virtfn &&
>> pci_dev->physfn->sriov->probe_vfs));
>>
>> We want to probe that device if 1) it's a PF 2) it'a VF and probe_vfs is set
>>> }
>>> #else
>>> static inline bool pci_device_can_probe(struct pci_dev *pdev)
>>> {
>>> 	return true;
>>> }
>> This function will be a waste if CONFIG_PCI_IOV is not defined.
>>> #endif
> It makes the code a bit clean. Nope, the proposed conditional
> expression is elaborate. Yeah, the purpose is exactly same as
> you said: probe driver for non-VF or VFs that were allowed.
>
>       (!pdev->is_virtfn || pdev->physfn->sriov->probe_vfs);
>
> When pdev->is_virtfn is flase, "pdev->physfn->sriov->probe_vfs"
> doesn't take effect. Otherwise, it means pdev->is_virtfn is true
> indirectly and going to check "pdev->physfn->sriov->probe_vfs".
> So it needn't check pdev->is_virtfn explicitly in later case,
> but it isn't wrong :)
>
> Thanks,
> Gavin
>
Make sense :) Will apply in V1.

Thanks,

Bodong