Thanks for the detailed review Bjorn!
> + Enabling this option will also disable ACS on all ports behind
> + any PCIe switch. This effictively puts all devices behind any
> + switch into the same IOMMU group.
Does this really mean "all devices behind the same Root Port"?
Not necessarily. You might have a cascade of switches (i.e switches below a switch) to
achieve a very large fan-out (in an NVMe SSD array for example) and we will only disable
ACS on the ports below the relevant switch.
What does this mean in terms of device security? I assume it
at least, that individual devices can't be assigned to separate VMs.
This was discussed during v1 . Disabling ACS on all downstream ports of the switch
means that all the EPs below it have to part of the same IOMMU grouping. However it was
also agreed that as long as the ACS disable occurred at boot time (which is does in v2)
then the virtualization layer will be aware of it and will perform the IOMMU group
I don't mind admitting that this patch makes me pretty nervous,
don't have a clear idea of what the implications of this are, or how
to communicate those to end users. "The same IOMMU group" is a pretty
Alex gave a good overview of the implications in .