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 16:27:21 +0800
From:	Wei Yang <weiyang@...ux.vnet.ibm.com>
To:	Alexander Duyck <alexander.duyck@...il.com>
Cc:	Wei Yang <weiyang@...ux.vnet.ibm.com>,
	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 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?

>Also this is an exception case.  There really isn't much point in optimizing
>for something that should never really happen.
>
>>>+
>>>+	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);
>>>+		rc = -ENOMEM;
>>>+		goto err_bus;
>>>+	}
>>>+
>>>	iov->ctrl |= PCI_SRIOV_CTRL_VFE | PCI_SRIOV_CTRL_MSE;
>>>	pci_cfg_access_lock(dev);
>>>	pci_write_config_word(dev, iov->pos + PCI_SRIOV_CTRL, iov->ctrl);
>>>@@ -342,7 +344,7 @@ err_pcibios:
>>>	pci_write_config_word(dev, iov->pos + PCI_SRIOV_CTRL, iov->ctrl);
>>>	ssleep(1);
>>>	pci_cfg_access_unlock(dev);
>>>-
>>>+err_bus:
>>>	if (iov->link != dev->devfn)
>>>		sysfs_remove_link(&dev->dev.kobj, "dep_link");
>>>
>
>--
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-- 
Richard Yang
Help you, Help me

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