linux-kernel - Re: [PATCH v10 1/7] PCI: initialize and release SR-IOV capability

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Date:	Fri, 6 Mar 2009 13:08:10 -0700
From:	Matthew Wilcox <matthew@....cx>
To:	Yu Zhao <yu.zhao@...el.com>
Cc:	jbarnes@...tuousgeek.org, linux-pci@...r.kernel.org,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	"Randy.Dunlap" <rdunlap@...otime.net>
Subject: Re: [PATCH v10 1/7] PCI: initialize and release SR-IOV capability

On Fri, Feb 20, 2009 at 02:54:42PM +0800, Yu Zhao wrote:
> +config PCI_IOV
> +	bool "PCI IOV support"
> +	depends on PCI
> +	select PCI_MSI

My understanding is that having 'select' of a config symbol that the
user can choose is bad.  I think we should probably make this 'depends
on PCI_MSI'.

PCI MSI can also be disabled at runtime (and Fedora do by default).
Since SR-IOV really does require MSI, we need to put in a runtime check
to see if pci_msi_enabled() is false.

We don't depend on PCIEPORTBUS (a horribly named symbol).  Should we?
SR-IOV is only supported for PCI Express machines.  I'm not sure of the
right answer here, but I thought I should raise the question.

> +	default n

You don't need this -- the default default is n ;-)

> +	help
> +	  PCI-SIG I/O Virtualization (IOV) Specifications support.
> +	  Single Root IOV: allows the Physical Function driver to enable
> +	  the hardware capability, so the Virtual Function is accessible
> +	  via the PCI Configuration Space using its own Bus, Device and
> +	  Function Numbers. Each Virtual Function also has the PCI Memory
> +	  Space to map the device specific register set.

I'm not convinced this is the most helpful we could be to the user who's
configuring their own kernel.  How about something like this?  (Randy, I
particularly look to you to make my prose less turgid).

	help
	  IO Virtualisation is a PCI feature supported by some devices
	  which allows you to create virtual PCI devices and assign them
	  to guest OSes.  This option needs to be selected in the host
	  or Dom0 kernel, but does not need to be selected in the guest
	  or DomU kernel.  If you don't know whether your hardware supports
	  it, you can check by using lspci to look for the SR-IOV capability.

	  If you have no idea what any of that means, it is safe to
	  answer 'N' here.

> diff --git a/drivers/pci/Makefile b/drivers/pci/Makefile
> index 3d07ce2..ba99282 100644
> --- a/drivers/pci/Makefile
> +++ b/drivers/pci/Makefile
> @@ -29,6 +29,9 @@ obj-$(CONFIG_DMAR) += dmar.o iova.o intel-iommu.o
>  
>  obj-$(CONFIG_INTR_REMAP) += dmar.o intr_remapping.o
>  
> +# PCI IOV support
> +obj-$(CONFIG_PCI_IOV) += iov.o

I see you're following the gerneal style in this file, but the comments
really add no value.  I should send a patch to take out the existing ones.

> +	list_for_each_entry(pdev, &dev->bus->devices, bus_list)
> +		if (pdev->sriov)
> +			break;
> +	if (list_empty(&dev->bus->devices) || !pdev->sriov)
> +		pdev = NULL;
> +	ctrl = 0;
> +	if (!pdev && pci_ari_enabled(dev->bus))
> +		ctrl |= PCI_SRIOV_CTRL_ARI;
> +

I don't like this loop.  At the end of a list_for_each_entry() loop,
pdev will not be pointing at a pci_device, it'll be pointing to some
offset from &dev->bus->devices.  So checking pdev->sriov at this point
is really, really bad.  I would prefer to see something like this:

        ctrl = 0;
        list_for_each_entry(pdev, &dev->bus->devices, bus_list) {
                if (pdev->sriov)
                        goto ari_enabled;
        }

        if (pci_ari_enabled(dev->bus))
                ctrl = PCI_SRIOV_CTRL_ARI;
 ari_enabled:
        pci_write_config_word(dev, pos + PCI_SRIOV_CTRL, ctrl);

> +	if (pdev)
> +		iov->pdev = pci_dev_get(pdev);
> +	else {
> +		iov->pdev = dev;
> +		mutex_init(&iov->lock);
> +	}

Now I'm confused.  Why don't we need to init the mutex if there's another
device on the same bus which also has an iov capability?

> +static void sriov_release(struct pci_dev *dev)
> +{
> +	if (dev == dev->sriov->pdev)
> +		mutex_destroy(&dev->sriov->lock);
> +	else
> +		pci_dev_put(dev->sriov->pdev);
> +
> +	kfree(dev->sriov);
> +	dev->sriov = NULL;
> +}

> +void pci_iov_release(struct pci_dev *dev)
> +{
> +	if (dev->sriov)
> +		sriov_release(dev);
> +}

This seems to be a bit of a design pattern with you, and I'm not quite sure why you do it like this instead of just doing:

void pci_iov_release(struct pci_dev *dev)
{
	if (!dev->sriov)
		return;
	[...]
}

-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."
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