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

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Message-Id: <20160701060519.GA8863@gwshan>
Date:	Fri, 1 Jul 2016 16:05:19 +1000
From:	Gavin Shan <gwshan@...ux.vnet.ibm.com>
To:	Yongji Xie <xyjxie@...ux.vnet.ibm.com>
Cc:	Gavin Shan <gwshan@...ux.vnet.ibm.com>, 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

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.

Thanks,
Gavin