On Fri, Jun 07, 2024 at 02:30:55PM -0500, Bjorn Helgaas wrote:

"Correctly" is not quite the right word here; it's just a fact that
the ACS settings determined at boot time result in certain IOMMU
groups.  If the user desires different groups, it's not that something
is "incorrect"; it's just that the user may have to accept less
isolation to get the desired IOMMU groups.

That is not quite accurate.. There are HW configurations where ACS
needs to be a certain way for the HW to work with P2P at all. It isn't
just an optimization or the user accepts something, if they want P2P
at all they must get a ACS configuration appropriate for their system.

Jason