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

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Date:	Fri, 1 Jul 2016 10:39:59 +1000
From:	Gavin Shan <gwshan@...ux.vnet.ibm.com>
To:	Yongji Xie <xyjxie@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org, linux-doc@...r.kernel.org,
	bhelgaas@...gle.com, alex.williamson@...hat.com, aik@...abs.ru,
	benh@...nel.crashing.org, paulus@...ba.org, mpe@...erman.id.au,
	corbet@....net, warrier@...ux.vnet.ibm.com,
	zhong@...ux.vnet.ibm.com, nikunj@...ux.vnet.ibm.com,
	gwshan@...ux.vnet.ibm.com
Subject: Re: [PATCH v3 2/7] PCI: Ignore enforced alignment to VF BARs

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?

> 	/* check if specified PCI is target device to reassign */
> 	align = pci_specified_resource_alignment(dev);
> 	if (!align)

Thanks,
Gavin