linux-kernel - Re: [PATCH v3 2/7] PCI: Ignore enforced alignment to VF BARs

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Date:	Fri, 1 Jul 2016 14:40:16 +0800
From:	Yongji Xie <xyjxie@...ux.vnet.ibm.com>
To:	Gavin Shan <gwshan@...ux.vnet.ibm.com>
Cc:	nikunj@...ux.vnet.ibm.com, zhong@...ux.vnet.ibm.com,
	linux-doc@...r.kernel.org, aik@...abs.ru,
	linux-pci@...r.kernel.org, corbet@....net,
	linux-kernel@...r.kernel.org, bhelgaas@...gle.com,
	alex.williamson@...hat.com, paulus@...ba.org,
	warrier@...ux.vnet.ibm.com, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v3 2/7] PCI: Ignore enforced alignment to VF BARs

Hi Gavin,

On 2016/7/1 14:05, Gavin Shan wrote:

> On Fri, Jul 01, 2016 at 01:27:17PM +0800, Yongji Xie wrote:
>>> On Thu, Jun 30, 2016 at 06:53:08PM +0800, Yongji Xie wrote:
>>>> VF BARs are read-only zeroes according to SRIOV spec,
>>>> the normal way(writing BARs) of allocating resources wouldn't
>>>> be applied to VFs. The VFs' resources would be allocated
>>>> when we enable SR-IOV capability. So we should not try to
>>>> reassign alignment after we enable VFs. It's meaningless
>>>> and will release the allocated resources which leads to a bug.
>>>>
>>>> Signed-off-by: Yongji Xie <xyjxie@...ux.vnet.ibm.com>
>>>> ---
>>>> drivers/pci/pci.c |    4 ++++
>>>> 1 file changed, 4 insertions(+)
>>>>
>>>> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
>>>> index be8f72c..6ae02de 100644
>>>> --- a/drivers/pci/pci.c
>>>> +++ b/drivers/pci/pci.c
>>>> @@ -4822,6 +4822,10 @@ void pci_reassigndev_resource_alignment(struct pci_dev *dev)
>>>> 	resource_size_t align, size;
>>>> 	u16 command;
>>>>
>>>> +	/* We should never try to reassign VF's alignment */
>>>> +	if (dev->is_virtfn)
>>>> +		return;
>>>> +
>>> Yongji, I think it's correct to ignore VF's BARs. Another concern is:
>>> it's safe to apply alignment to PF's IOV BARs? Lets have an extreme
>>> example here: one PF has 16 VFs; each VF has only one 1KB. It means
>>> the only PF IOV BAR is 16KB. I don't see how it works after expanding
>>> it to 64KB which is the page size. It might be not a problem on PowerNV
>>> platform, but potentially a issue on x86?
>> Seems like the alignment would not be applied to IOV BARs because
>> pci_reassigndev_resource_alignment() will be called before
>> sriov_init().
>>
> Correct, thanks for the claim. I guess the alignment applied to PF IOV
> BARs should be ignored as well? Anyway, the IOV BARs are retireved from
> SRIOV capability. It deserves a comment if you plan to take the change.
> Actually, the comment here (for ignoring alignment to VF BARs) can be
> improved a bit as well, it'd better why the alignment cannot be applied.
>

Do you mean we should ignore PF IOV BARs like this:

--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -4833,7 +4833,7 @@ void pci_reassigndev_resource_alignment(struct 
pci_dev *dev)
         command &= ~PCI_COMMAND_MEMORY;
         pci_write_config_word(dev, PCI_COMMAND, command);

-       for (i = 0; i < PCI_BRIDGE_RESOURCES; i++) {
+       for (i = 0; i <= PCI_ROM_RESOURCE; i++) {
                 r = &dev->resource[i];
                 if (!(r->flags & IORESOURCE_MEM))
                         continue;

Thanks,
Yongji