Re: [PATCH 04/11] PCI: portdrv: Suppress kernel DMA ownership auto-claiming

[Bjorn Helgaas]

On Tue, Nov 16, 2021 at 03:24:29PM +0800, Lu Baolu wrote:
> On 2021/11/16 4:44, Bjorn Helgaas wrote:
> > On Mon, Nov 15, 2021 at 10:05:45AM +0800, Lu Baolu wrote:
> > > IOMMU grouping on PCI necessitates that if we lack isolation on a bridge
> > > then all of the downstream devices will be part of the same IOMMU group
> > > as the bridge.
> > 
> > I think this means something like: "If a PCIe Switch Downstream Port
> > lacks <a specific set of ACS capabilities>, all downstream devices
> > will be part of the same IOMMU group as the switch," right?
> 
> For this patch, yes.
> 
> > If so, can you fill in the details to make it specific and concrete?
> 
> The existing vfio implementation allows a kernel driver to bind with a
> PCI bridge while its downstream devices are assigned to the user space
> though there lacks ACS-like isolation in bridge.
> 
> drivers/vfio/vfio.c:
>  540 static bool vfio_dev_driver_allowed(struct device *dev,
>  541                                     struct device_driver *drv)
>  542 {
>  543         if (dev_is_pci(dev)) {
>  544                 struct pci_dev *pdev = to_pci_dev(dev);
>  545
>  546                 if (pdev->hdr_type != PCI_HEADER_TYPE_NORMAL)
>  547                         return true;
>  548         }
> 
> We are moving the group viability check to IOMMU core, and trying to
> make it compatible with the current vfio policy. We saw three types of
> bridges:
> 
> #1) PCIe/PCI-to-PCI bridges
>     These bridges are configured in the PCI framework, there's no
>     dedicated driver for such devices.
> 
> #2) Generic PCIe switch downstream port
>     The port driver doesn't map and access any MMIO in the PCI BAR.
>     The iommu group is viable to user even this driver is bound.
> 
> #3) Hot Plug Controller
>     The controller driver maps and access the device MMIO. The iommu
>     group is not viable to user with this driver bound to its device.

I *guess* the question here is whether the bridge can or will do DMA?

I think that's orthogonal to the question of whether it implements
BARs, so I'm not sure why the MMIO BARs are part of this discussion.
I assume it's theoretically possible for a driver to use registers in
config space to program a device to do DMA, even if the device has no
BARs.

Bjorn