On Thu, Jun 08, 2023 at 07:33:31AM -0700, Alexander Duyck wrote: > On Wed, Jun 7, 2023 at 8:05 PM Baolu Lu <baolu.lu@xxxxxxxxxxxxxxx> wrote: > > > > On 6/8/23 7:03 AM, Alexander Duyck wrote: > > > On Wed, Jun 7, 2023 at 3:40 PM Alexander Duyck > > > <alexander.duyck@xxxxxxxxx> wrote: > > >> > > >> I am running into a DMA issue that appears to be a conflict between > > >> ACS and IOMMU. As per the documentation I can find, the IOMMU is > > >> supposed to create reserved regions for MSI and the memory window > > >> behind the root port. However looking at reserved_regions I am not > > >> seeing that. I only see the reservation for the MSI. > > >> > > >> So for example with an enabled NIC and iommu enabled w/o passthru I am seeing: > > >> # cat /sys/bus/pci/devices/0000\:83\:00.0/iommu_group/reserved_regions > > >> 0x00000000fee00000 0x00000000feefffff msi > > >> > > >> Shouldn't there also be a memory window for the region behind the root > > >> port to prevent any possible peer-to-peer access? > > > > > > Since the iommu portion of the email bounced I figured I would fix > > > that and provide some additional info. > > > > > > I added some instrumentation to the kernel to dump the resources found > > > in iova_reserve_pci_windows. From what I can tell it is finding the > > > correct resources for the Memory and Prefetchable regions behind the > > > root port. It seems to be calling reserve_iova which is successfully > > > allocating an iova to reserve the region. > > > > > > However still no luck on why it isn't showing up in reserved_regions. > > > > Perhaps I can ask the opposite question, why it should show up in > > reserve_regions? Why does the iommu subsystem block any possible peer- > > to-peer DMA access? Isn't that a decision of the device driver. > > > > The iova_reserve_pci_windows() you've seen is for kernel DMA interfaces > > which is not related to peer-to-peer accesses. > > The problem is if the IOVA overlaps with the physical addresses of > other devices that can be routed to via ACS redirect. As such if ACS > redirect is enabled a host IOVA could be directed to another device on > the switch instead. To prevent that we need to reserve those addresses > to avoid address space collisions. Any untranslated address from a device must be forwarded to the IOMMU when ACS is enabled correct? I guess if you want true p2p, then you would need to map so that the hpa turns into the peer address.. but its always a round trip to IOMMU. > > From what I can tell it looks like the IOVA should be reserved, but I > don't see it showing up anywhere in reserved_regions. What I am > wondering is if iova_reserve_pci_windows() should be taking some steps > so that it will appear, or if intel_iommu_get_resv_regions() needs to > have some code similar to iova_reserve_pci_windows() to get the ranges > and verify they are reserved in the IOVA. >
