Messages in this thread | | | From | "Tian, Kevin" <> | Subject | RE: [PATCH v10 04/12] iommu: Add attach/detach_dev_pasid iommu interface | Date | Wed, 27 Jul 2022 03:20:25 +0000 |
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> From: Jason Gunthorpe <jgg@nvidia.com> > Sent: Tuesday, July 26, 2022 9:57 PM > > 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 like ACS enables PASID-based routing...
In reality PCI fabric always route TLP based on memory address. ACS just provides a way to redirect the packet to RC, with or without PASID.
So it's simply that PASID requires such redirection hence ACS because only RC/IOMMU understands PASID and related address space.
> > > + /* > > + * 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.
But there is no single line in above check related to PASID...
> > If the fabric routes PASID properly then groups are not an issue - all > agree on this? >
IMHO if the fabric can route PASID properly, and according to above once such redirect is available it applies to both non-PASID and PASID TLP, then the group will be singleton in the first place.
Is there a real-world example where the fabric can route PASID properly for a multi-devices group?
Thanks Kevin
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