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

[Yongji Xie]

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@xxxxxxxxxxxxxxxxxx>
> > > > ---
> > > > 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