24-28 August 2020
US/Pacific timezone

Passthrough of VMD subdevices

26 Aug 2020, 10:25
15m
Microconference2/Virtual-Room (LPC 2020)

Microconference2/Virtual-Room

LPC 2020

150
VFIO/IOMMU/PCI MC VFIO/IOMMU/PCI MC

Speaker

Jonathan Derrick

Description

The Intel Volume Management Device (VMD) behaves similar to a PCI-to-PCI bridge that changes the subdevice's requester ID to VMD's. VMD also remaps subdevice MSI/X into its own MSI/X mapping table. Because of the requester ID factor, the VMD device and subdevice domain fall under a single IOMMU group.

VMD is being integrated more and more into Intel chipsets and the desire to assign individual subdevices is only going to become more of an outstanding problem as time goes on. The existing model of assignment of the whole IOMMU group to a VM is problematic to VMD subdevices, as well as any expectation surrounding interrupt remapping.

VFIO/IOMMU may need an (unsafe) DMA remapping provider-consumer relationship to assigning individual subdevice DMA contexts. To handle MSI/X, the guest may need to avoid using it in the first place, or have VMD in the host deliver the interrupts.

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Primary author

Presentation Materials