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:

quoted

quoted

On Thu, Jun 30, 2016 at 06:53:08PM +0800, Yongji Xie wrote:

quoted

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 <redacted>
---
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