Re: Question about reserved_regions w/ Intel IOMMU

[Ashok Raj <ashok_raj@linux.intel.com>]

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@linux.intel.com> wrote:
> >
> > On 6/8/23 7:03 AM, Alexander Duyck wrote:
> > > On Wed, Jun 7, 2023 at 3:40 PM Alexander Duyck
> > > <alexander.duyck@gmail.com> 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.
>