Re: [PATCH 0/2] Introduce PCI_FIXUP_IOMMU

[Bjorn Helgaas <helgaas@kernel.org>]

On Mon, Jun 08, 2020 at 10:54:15AM +0800, Zhangfei Gao wrote:
> On 2020/6/6 上午7:19, Bjorn Helgaas wrote:
> > On Thu, Jun 04, 2020 at 09:33:07PM +0800, Zhangfei Gao wrote:
> > > On 2020/6/2 上午1:41, 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,
> > >          fwspec->iommu_fwnode = iommu_fwnode;
> > >          fwspec->ops = ops;
> > >          dev_iommu_fwspec_set(dev, fwspec);
> > > +
> > > +       if (dev_is_pci(dev))
> > > +               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.
> >
> > Wait, this whole fixup approach seems wrong to me.  No matter how you
> > do the fixup, it's still a fixup, which means it requires ongoing
> > maintenance.  Surely we don't want to have to add the Vendor/Device ID
> > for every new AMBA device that comes along, do we?
> > 
> Here the fake pci device has standard PCI cfg space, but physical
> implementation is base on AMBA
> They can provide pasid feature.
> However,
> 1, does not support tlp since they are not real pci devices.
> 2. does not support pri, instead support stall (provided by smmu)
> And stall is not a pci feature, so it is not described in struct pci_dev,
> but in struct iommu_fwspec.
> So we use this fixup to tell pci system that the devices can support stall,
> and hereby support pasid.

This did not answer my question.  Are you proposing that we update a
quirk every time a new AMBA device is released?  I don't think that
would be a good model.