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

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Date:	Mon, 2 Nov 2015 07:46:24 -0800
From:	Alexander Duyck <alexander.duyck@...il.com>
To:	Wei Yang <weiyang@...ux.vnet.ibm.com>
Cc:	ethan zhao <ethan.zhao@...cle.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	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 11/02/2015 12:33 AM, Wei Yang wrote:
> On Fri, Oct 30, 2015 at 08:57:17AM -0700, Alexander Duyck wrote:
>> 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.
>>
> Yep, I think the statement is met, "NumVFs may only be written while VF Enable
> is Clear".
>
> While in your commit log, the purpose of this patch is to wait 1 second before
> write NumVFs. So I am interesting to know why you move this out of the
> pci_cfg_access_lock. Because it looks better? have better performance?
>
> Actually, this is a question instead of a challenge :-)

It is because the first call to pci_iov_set_numvfs is done outside of 
the pci_cfg_access_lock.  This way when I add the clean-up for the bus 
numbering failure in patch 7 I don't have to modify as much code either 
since the write is already pulled out.

An added bonus is the code is now much closer to what we have in 
sriov_disable which has seen much more use than the exception handling 
case for sriov_enable, so it has been more thoroughly tested.

- Alex

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