port-amd64/58308: x86 bus_dmamem_map lacks BUS_DMA_PREFETCHABLE support

[campbell+netbsd%mumble.net@localhost]

>Number:         58308
>Category:       port-amd64
>Synopsis:       x86 bus_dmamem_map lacks BUS_DMA_PREFETCHABLE support
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    port-amd64-maintainer
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Tue Jun 04 21:30:01 +0000 2024
>Originator:     Taylor R Campbell
>Release:        current, 10, 9, ...
>Organization:
The NetDMA Prefoundation
>Environment:
x86
>Description:
x86 bus_dmamem_map ignores BUS_DMA_PREFETCHABLE, so the mapping is normal (write-back, cached) rather than prefetchable (write-combining but not cached).
>How-To-Repeat:
code inspection
>Fix:
Yes, please!