Re: [PATCH 0/2] Introduce PCI_FIXUP_IOMMU

[Bjorn Helgaas <helgaas@kernel.org>]

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.

Bjorn

[1] https://lore.kernel.org/linux-pci/b1533fd5-1fae-7256-9597-36d3d5de9d2a@molgen.mpg.de/