Re: [PATCH 0/2] Introduce PCI_FIXUP_IOMMU

From: Zhangfei Gao
Date: Thu Jun 04 2020 - 09:33:36 EST

On 2020/6/2 äå1:41, Bjorn Helgaas wrote:
On Thu, May 28, 2020 at 09:33:44AM +0200, Joerg Roedel wrote:
On Wed, May 27, 2020 at 01:18:42PM -0500, Bjorn Helgaas wrote:
Is this slowdown significant? We already iterate over every device
when applying PCI_FIXUP_FINAL quirks, so if we used the existing
PCI_FIXUP_FINAL, we wouldn't be adding a new loop. We would only be
adding two more iterations to the loop in pci_do_fixups() that tries
to match quirks against the current device. I doubt that would be a
measurable slowdown.
I don't know how significant it is, but I remember people complaining
about adding new PCI quirks because it takes too long for them to run
them all. That was in the discussion about the quirk disabling ATS on
AMD Stoney systems.

So it probably depends on how many PCI devices are in the system whether
it causes any measureable slowdown.
I found this [1] from Paul Menzel, which was a slowdown caused by
quirk_usb_early_handoff(). I think the real problem is individual
quirks that take a long time.

The PCI_FIXUP_IOMMU things we're talking about should be fast, and of
course, they're only run for matching devices anyway. So I'd rather
keep them as PCI_FIXUP_FINAL than add a whole new phase.

Thanks Bjorn for taking time for this.
If so, it would be much simpler.

+++ b/drivers/iommu/iommu.c
@@ -2418,6 +2418,10 @@ int iommu_fwspec_init(struct device *dev, struct fwnode_handle *iommu_fwnode,
ÂÂÂÂÂÂÂ fwspec->iommu_fwnode = iommu_fwnode;
ÂÂÂÂÂÂÂ fwspec->ops = ops;
ÂÂÂÂÂÂÂ dev_iommu_fwspec_set(dev, fwspec);
+ÂÂÂÂÂÂ if (dev_is_pci(dev))
+ÂÂÂÂÂÂÂÂÂÂÂÂÂÂ pci_fixup_device(pci_fixup_final, to_pci_dev(dev));

Then pci_fixup_final will be called twice, the first in pci_bus_add_device.
Here in iommu_fwspec_init is the second time, specifically for iommu_fwspec.
Will send this when 5.8-rc1 is open.