linux-kernel - Re: [PATCH v2 7/7] PCI: Set NumVFs before computing how many buses VFs require

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Date:	Mon, 2 Nov 2015 07:39:48 -0800
From:	Alexander Duyck <alexander.duyck@...il.com>
To:	Wei Yang <weiyang@...ux.vnet.ibm.com>
Cc:	Bjorn Helgaas <bhelgaas@...gle.com>,
	Alexander Duyck <aduyck@...antis.com>,
	linux-pci@...r.kernel.org, Ethan Zhao <ethan.zhao@...cle.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 7/7] PCI: Set NumVFs before computing how many buses
 VFs require

On 11/02/2015 12:27 AM, Wei Yang wrote:
> On Fri, Oct 30, 2015 at 09:03:54AM -0700, Alexander Duyck wrote:
>> On 10/29/2015 10:22 PM, Wei Yang wrote:
>>> On Thu, Oct 29, 2015 at 05:23:36PM -0500, Bjorn Helgaas wrote:
>>>> From: Alexander Duyck <aduyck@...antis.com>
>>>>
>>>> VF bus numbers depend on the First VF Offset and VF Stride, and per
>>>> sections 3.3.9 and 3.3.10 of the SR-IOV spec r1.1, these depend on the
>>>> NumVF value.
>>>>
>>>> Wait until after we set NumVFs to compute and validate the bus number of
>>>> the last VF.
>>>>
>>>> [bhelgaas: changelog, add spec reference, split to separate patch for
>>>> reviewability]
>>>> Signed-off-by: Alexander Duyck <aduyck@...antis.com>
>>>> Signed-off-by: Bjorn Helgaas <bhelgaas@...gle.com>
>>>> ---
>>>> drivers/pci/iov.c |   18 ++++++++++--------
>>>> 1 file changed, 10 insertions(+), 8 deletions(-)
>>>>
>>>> diff --git a/drivers/pci/iov.c b/drivers/pci/iov.c
>>>> index bd1c4fa..9d29712 100644
>>>> --- a/drivers/pci/iov.c
>>>> +++ b/drivers/pci/iov.c
>>>> @@ -274,13 +274,6 @@ static int sriov_enable(struct pci_dev *dev, int nr_virtfn)
>>>> 		return -ENOMEM;
>>>> 	}
>>>>
>>>> -	bus = pci_iov_virtfn_bus(dev, nr_virtfn - 1);
>>>> -	if (bus > dev->bus->busn_res.end) {
>>>> -		dev_err(&dev->dev, "can't enable %d VFs (bus %02x out of range of %pR)\n",
>>>> -			nr_virtfn, bus, &dev->bus->busn_res);
>>>> -		return -ENOMEM;
>>>> -	}
>>>> -
>>>> 	if (pci_enable_resources(dev, bars)) {
>>>> 		dev_err(&dev->dev, "SR-IOV: IOV BARS not allocated\n");
>>>> 		return -ENOMEM;
>>>> @@ -304,6 +297,15 @@ static int sriov_enable(struct pci_dev *dev, int nr_virtfn)
>>>> 	}
>>>>
>>>> 	pci_iov_set_numvfs(dev, nr_virtfn);
>>> How about move it up?
>>
>> The idea with moving the write down is to keep the pollution of the SR-IOV
>> capability to a minimum.  Basically we have addressed all of the possible
>> software issues at this point so all that remains is possible hardware
>> complications.  In addition by moving this code down we only have to modify
>> this code instead of adding "rc=X; goto foo;" in places where "return X;" was
>> used.
>>
>
> I think your logic is clear, while it is not easy to classify the software
> issue and hardware complications. For example, at the beginning of
> sriov_enable(), the hardware value initial VFs number is checked.
>
> And in my mind, this is reasonable to check the hardware issue before software
> issue.
>
> For your comment, adding "rc=X; goto foo;", I don't see this would happen.
> The code in my mind would like this:
>
>
> +	pci_iov_set_numvfs(dev, nr_virtfn);
>   	bus = pci_iov_virtfn_bus(dev, nr_virtfn - 1);
>   	if (bus > dev->bus->busn_res.end) {
>   		dev_err(&dev->dev, "can't enable %d VFs (bus %02x out of range of %pR)\n",
>   			nr_virtfn, bus, &dev->bus->busn_res);
>   		return -ENOMEM;
>   	}
>
> 	if (pci_enable_resources(dev, bars)) {
> 		dev_err(&dev->dev, "SR-IOV: IOV BARS not allocated\n");
> 		return -ENOMEM;
>
> Do I missed something?

The problem is that pci_iov_set_numvfs has side effects visible to the 
user since they can read NumVFs lspci and via sysfs.  As such we want to 
keep the two in sync, and if you get the bus error here that is not the 
case.

That is why you must call pci_iov_set_numvfs(dev, 0) if this fails.

- Alex

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