Re: [PATCH 0/2] Introduce PCI_FIXUP_IOMMU

Bjorn Helgaas
2020-06-01T17:41:11Z

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/