> -----Original Message----- > From: linux-pci-owner@xxxxxxxxxxxxxxx [mailto:linux-pci- > owner@xxxxxxxxxxxxxxx] On Behalf Of Bjorn Helgaas > Sent: Tuesday, January 19, 2016 10:39 PM > To: Alex Williamson <alex.williamson@xxxxxxxxxx> > Cc: Lawrynowicz, Jacek <jacek.lawrynowicz@xxxxxxxxx>; linux- > pci@xxxxxxxxxxxxxxx; bhelgaas@xxxxxxxxxx; dwmw2@xxxxxxxxxxxxx; > jroedel@xxxxxxx > Subject: Re: [PATCH] pci: Add support for multiple DMA aliases > > On Tue, Jan 19, 2016 at 02:04:31PM -0700, Alex Williamson wrote: > > On Tue, 2016-01-19 at 14:12 -0600, Bjorn Helgaas wrote: > > > [+cc Alex] > > > > > > On Mon, Jan 18, 2016 at 09:33:15PM -0600, Bjorn Helgaas wrote: > > > > On Mon, Jan 18, 2016 at 05:07:47PM +0100, Jacek Lawrynowicz wrote: > > > > > This patch solves IOMMU support issues with PCIe non-transparent > > > > > bridges that use Requester ID look-up tables (LUT), e.g. > > > > > PEX8733. Before exiting the bridge, packet's RID is rewritten > > > > > according to LUT programmed by a driver. Modified packets are > > > > > then passed to a destination bus and processed upstream. The > > > > > problem is that such packets seem to come from non-existent > > > > > nodes that are hidden behind NTB and are not discoverable by a > > > > > destination node, so IOMMU discards them. Adding DMA alias for a > > > > > given LUT entry allows IOMMU to create a proper mapping that > enables inter-node communication. > > > > > > > > > > The current DMA alias implementation supports only single alias, > > > > > so it's not possible to connect more than two nodes when IOMMU > > > > > is enabled. This implementation enables all possible aliases on > > > > > a given bus (256) that are stored in a bitset. Alias devfn is > > > > > directly translated to a bit number. The bitset is not allocated > > > > > for devices that have no need for DMA aliases. > > > > My only concern here is that pci_add_dma_alias() makes aliases seem > > more dynamic than they really are. For instance, when we add a device > > to an IOMMU domain, we evaluate the aliases at that point, if an NTB > > later adds a new lookup entry and specifies a new alias, it's still > > not going to work. Similarly, IOMMU groups are evaluated as the > > device is added, so if an alias is to a physical device and we need > > the cross reference to bind them together into a single group, calling > > pci_add_dma_alias() from a driver isn't going to work. > > > > The existing code had this problem too, it's just more obvious now > > that we have a helper function and that the helper is exported for use > > outside of the PCI core. Thanks, > > Oh, that's a really good point. I hadn't noticed the export. Is there any > reason pci_add_dma_alias() needs to be declared in include/linux/pci.h and > exported to modules? > > I don't think the current patch requires the export, but I suppose you > envision an NTB driver that might be a module? I guess we can easily export > it when that driver is merged if that seems the best solution. This export would be useful for Xeon Phi x200 which uses on a NTB generating multiple RIDs. x200 is not yet ready for upstreming (x100 is already upstreamed) and having this export would make driver development less painful. -- Jacek Lawrynowicz Intel Technology Poland sp. z o.o. KRS 101882 - ul. Slowackiego 173, 80-298 Gdansk -- To unsubscribe from this list: send the line "unsubscribe linux-pci" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
