Re: [PATCH 2/2] NTB: PCI Quirk to Enable Switchtec NT Functionality with IOMMU On

[Alex Williamson <alex.williamson@redhat.com>]

On Tue, 22 May 2018 16:51:26 -0500
Bjorn Helgaas <helgaas@kernel.org> wrote:

> [+cc Alex]
> 
> On Tue, May 22, 2018 at 02:09:59PM -0700, Doug Meyer wrote:
> > Logan answered the questions quite thoroughly. (Thanks, Logan!)  
> 
> When you repost it, please rework the commit log so it answers the
> questions directly.  Otherwise the next reader may have the same
> questions again.  E.g., say something about how the proxy IDs are not
> programmable and are fixed in the hardware so all we have to do is
> read them.
> 
> I don't think the question of when the aliases need to be added is
> quite closed.  Logan said "it seems pci_add_dma_alias() must be called
> before the driver is initialized and therefore in a quirk", but that
> doesn't make clear *why* the alias needs to be added before the driver
> is initialized.  The alias shouldn't be needed until the device does a
> DMA, and it shouldn't do that until after the driver initializes.

Aliases for devices that don't have a representation on the bus is only
one use for pci_add_dma_alias(), we can also use it when the aliased
device is visible on the bus and then it factors not only into the IOMMU
context entries for a given device, but also the grouping of multiple
devices that must be done without a host endpoint driver.
 
> I suspect the reason the existing quirks are in drivers/pci/quirks.c
> is because the IOMMU driver is in the host OS, but the host may not
> have a driver for the device if the device is passed through to a
> guest OS.  In that case, the only way to add the alias is by using a
> quirk that is always built into the host OS.
> 
> We could argue that the driver in the guest should be able to tell the
> host's IOMMU driver about these aliases, but I doubt there's an
> interface for that.

Sounds like a dangerous interface, imagine two physical functions on a
device, each assigned to separate guests where one guest could usurp
context entries for hidden devices from the other guest.  Thanks,

Alex