linux-kernel - Re: [PATCH v2 5/7] PCI: Wait 1 second between disabling VFs and clearing NumVFs

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Date:	Fri, 30 Oct 2015 08:57:17 -0700
From:	Alexander Duyck <alexander.duyck@...il.com>
To:	ethan zhao <ethan.zhao@...cle.com>,
	Wei Yang <weiyang@...ux.vnet.ibm.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>
Cc:	Alexander Duyck <aduyck@...antis.com>, linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 5/7] PCI: Wait 1 second between disabling VFs and
 clearing NumVFs

On 10/29/2015 11:00 PM, ethan zhao wrote:
> Wei,
>
> On 2015/10/30 13:14, Wei Yang wrote:
>> On Thu, Oct 29, 2015 at 05:23:22PM -0500, Bjorn Helgaas wrote:
>>> From: Alexander Duyck <aduyck@...antis.com>
>>>
>>> Per sec 3.3.3.1 of the SR-IOV spec, r1.1, we must allow 1.0s after 
>>> clearing
>>> VF Enable before reading any field in the SR-IOV Extended Capability.
>>>
>>> Wait 1 second before calling pci_iov_set_numvfs(), which reads
>>> PCI_SRIOV_VF_OFFSET and PCI_SRIOV_VF_STRIDE after it sets 
>>> PCI_SRIOV_NUM_VF.
>>>
>>> [bhelgaas: split to separate patch for reviewability, add spec 
>>> reference]
>>> Signed-off-by: Alexander Duyck <aduyck@...antis.com>
>>> Signed-off-by: Bjorn Helgaas <bhelgaas@...gle.com>
>>> ---
>>> drivers/pci/iov.c |    2 +-
>>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/pci/iov.c b/drivers/pci/iov.c
>>> index fada98d..24428d5 100644
>>> --- a/drivers/pci/iov.c
>>> +++ b/drivers/pci/iov.c
>>> @@ -339,13 +339,13 @@ failed:
>>>     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);
>>> -    pci_iov_set_numvfs(dev, 0);
>>>     ssleep(1);
>>>     pci_cfg_access_unlock(dev);
>>>
>>>     if (iov->link != dev->devfn)
>>>         sysfs_remove_link(&dev->dev.kobj, "dep_link");
>>>
>>> +    pci_iov_set_numvfs(dev, 0);
>> One small question, any specific reason put it here instead of just 
>> after
>> sleep()?
>  Agree,  pci_iov_set_numvfs(dev, 0) should be put before 
> pci_cfg_access_unlock(dev) to avoid race,  because "NumVFs may only be 
> written while VF Enable is Clear"

We are already guaranteeing that aren't we?  I'm assuming there is 
already code in place here somewhere that prevents us from both enabling 
and disabling SR-IOV from more than one thread.  Otherwise how could we 
hope to have any sort of consistent state?

I'm fine with us being more explicit about it if we want to be, but if 
we are going to do it we should probably update all 3 spots where we 
update NumVFs after init instead of just this one.  Perhaps it should be 
a separate patch.

- Alex
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