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

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Message-Id: <21112bd7-d343-46b6-1df1-b20cc1c1b24e@linux.vnet.ibm.com>
Date:	Fri, 1 Jul 2016 13:27:17 +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, warrier@...ux.vnet.ibm.com,
	alex.williamson@...hat.com, paulus@...ba.org, bhelgaas@...gle.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 8:39, Gavin Shan 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().

Thanks,
Yongji