Messages in this thread | | | Date | Thu, 28 Jul 2022 10:44:52 +0800 | Subject | Re: [PATCH v10 04/12] iommu: Add attach/detach_dev_pasid iommu interface | From | Baolu Lu <> |
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On 2022/7/26 21:57, Jason Gunthorpe wrote: > On Tue, Jul 26, 2022 at 02:23:26PM +0800, Baolu Lu wrote: >> On 2022/7/25 22:40, Jason Gunthorpe wrote: >>> On Sun, Jul 24, 2022 at 03:03:16PM +0800, Baolu Lu wrote: >>> >>>> How about rephrasing this part of commit message like below: >>>> >>>> Some buses, like PCI, route packets without considering the PASID value. >>>> Thus a DMA target address with PASID might be treated as P2P if the >>>> address falls into the MMIO BAR of other devices in the group. To make >>>> things simple, these interfaces only apply to devices belonging to the >>>> singleton groups. >>> >>>> Considering that the PCI bus supports hot-plug, even a device boots with >>>> a singleton group, a later hot-added device is still possible to share >>>> the group, which breaks the singleton group assumption. In order to >>>> avoid this situation, this interface requires that the ACS is enabled on >>>> all devices on the path from the device to the host-PCI bridge. >>> >>> But ACS directly fixes the routing issue above >>> >>> This entire explanation can be recast as saying we block PASID >>> attachment in all cases where the PCI fabric is routing based on >>> address. ACS disables that. >>> >>> Not sure it even has anything to do with hotplug or singleton?? >> >> Yes, agreed. I polished this patch like below. Does it look good to you? >> >> iommu: Add attach/detach_dev_pasid iommu interface >> >> Attaching an IOMMU domain to a PASID of a device is a generic operation >> for modern IOMMU drivers which support PASID-granular DMA address >> translation. Currently visible usage scenarios include (but not limited): >> >> - SVA (Shared Virtual Address) >> - kernel DMA with PASID >> - hardware-assist mediated device >> >> This adds a pair of domain ops for this purpose and adds the interfaces >> for device drivers to attach/detach a domain to/from a {device, >> PASID}. > >> The PCI bus routes packets without considering the PASID value. > > More like: > > Some configurations of the PCI fabric will route device originated TLP > packets based on memory address, and these configurations are > incompatible with PASID as the PASID packets form a distinct address > space. For instance any configuration where switches are present > without ACS is incompatible with PASID.
This description reads more accurate and professional. Thank you! I will update the patch with this.
> >> + /* >> + * Block PASID attachment in all cases where the PCI fabric is >> + * routing based on address. ACS disables it. >> + */ >> + if (dev_is_pci(dev) && >> + !pci_acs_path_enabled(to_pci_dev(dev), NULL, REQ_ACS_FLAGS)) >> + return -ENODEV; > > I would probably still put this in a function just to be clear, and > probably even a PCI layer funcion 'pci_is_pasid_supported' that > clearly indicates that the fabric path can route a PASID packet > without mis-routing it.
Fair enough. Let's keep this in iommu for now and leave the possible merging in PCI subsystem a future task.
> > If the fabric routes PASID properly then groups are not an issue - all > agree on this?
I still think the singleton group is required, but it's not related to the PCI fabric routing discussed here.
We have a single array for PASIDs in the iommu group. All devices sitting in the group should share a single PASID namespace. However both the translation structures for IOMMU hardware or the device drivers can only adapt to per-device PASID namespace. Hence, it's reasonable to require the singleton group.
Best regards, baolu
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