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

From: Wei Yang
Date: Mon Nov 02 2015 - 03:34:06 EST

On Fri, Oct 30, 2015 at 08:57:17AM -0700, Alexander Duyck wrote:
>On 10/29/2015 11:00 PM, ethan zhao wrote:
>>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@xxxxxxxxxxxx>
>>>>Per sec of the SR-IOV spec, r1.1, we must allow 1.0s after
>>>>VF Enable before reading any field in the SR-IOV Extended Capability.
>>>>Wait 1 second before calling pci_iov_set_numvfs(), which reads
>>>>[bhelgaas: split to separate patch for reviewability, add spec
>>>>Signed-off-by: Alexander Duyck <aduyck@xxxxxxxxxxxx>
>>>>Signed-off-by: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
>>>>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
>> 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 :-)

>- Alex

Richard Yang
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