Messages in this thread | | | Subject | Re: [PATCH v4 04/14] PCI/P2PDMA: Clear ACS P2P flags for all devices behind switches | From | Don Dutile <> | Date | Wed, 9 May 2018 11:47:31 -0400 |
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On 05/08/2018 08:01 PM, Alex Williamson wrote: > On Tue, 8 May 2018 19:06:17 -0400 > Don Dutile <ddutile@redhat.com> wrote: >> On 05/08/2018 05:27 PM, Stephen Bates wrote: >>> As I understand it VMs need to know because VFIO passes IOMMU >>> grouping up into the VMs. So if a IOMMU grouping changes the VM's >>> view of its PCIe topology changes. I think we even have to be >>> cognizant of the fact the OS running on the VM may not even support >>> hot-plug of PCI devices. >> Alex: >> Really? IOMMU groups are created by the kernel, so don't know how >> they would be passed into the VMs, unless indirectly via PCI(e) >> layout. At best, twiddling w/ACS enablement (emulation) would cause >> VMs to see different IOMMU groups, but again, VMs are not the >> security point/level, the host/HV's are. > > Correct, the VM has no concept of the host's IOMMU groups, only the > hypervisor knows about the groups, but really only to the extent of > which device belongs to which group and whether the group is viable. > Any runtime change to grouping though would require DMA mapping > updates, which I don't see how we can reasonably do with drivers, > vfio-pci or native host drivers, bound to the affected devices. Thanks, > > Alex > A change in iommu groups would/could require a device remove/add cycle to get an updated DMA-mapping (yet-another-overused-term: iommu 'domain').
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