[Bug 274252] sys/vm: less-than-ideal handling of memory requests that cannot be fulfilled

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=274252

            Bug ID: 274252
           Summary: sys/vm: less-than-ideal handling of memory requests
                    that cannot be fulfilled
           Product: Base System
           Version: Unspecified
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs@FreeBSD.org
          Reporter: kevans@freebsd.org

Splitting off from PR 274237, because I haven't actually created a PR for this
previously, but it'd be nice to track.

With at least some ARM machines, it's possible to get stuck in a nice loop in
xhci attach because the VM bits don't handle some class of requests that cannot
be satisfied very well. In particular, consider this system:

Physical memory chunk(s):                                                       
0x000008010a8000 - 0x00000802313fff, 19316736 bytes (1179 pages)                
0x000008023d8000 - 0x0000080389bfff, 21774336 bytes (1329 pages)                
0x000008038b8000 - 0x00000808f97fff, 91095040 bytes (5560 pages)                
0x00000808fb8000 - 0x0000080ba03fff, 44351488 bytes (2707 pages)                
0x0000080c12c000 - 0x000009d036ffff, 7585677312 bytes (462993 pages)            
0x000009d4f68000 - 0x000009db93bfff, 110968832 bytes (6773 pages)               
0x000009db944000 - 0x000009e096ffff, 84066304 bytes (5131 pages)                
0x000009e0980000 - 0x000009e0a37fff, 753664 bytes (46 pages)                    
avail memory = 7955300352 (7586 MB)                                             

Note that there's absolutely no RAM in the lower 4G of the address space.
There's an XHCI controller that can only do 32-bit DMA (allegedly) and it has
an associated IOMMU that isn't hooked up just yet.

Right now, busdma will request some pages below 4G (IIRC, it's with
kmem_alloc_contig here[0]), but that request cannot be satisfied -- there's
absolutely no memory there. Instead, it ends up hanging in the VM layer trying
to fulfill an allocation that isn't physically possible.

I think it'd be better to fail the request and let busdma kick back an ENOMEM.
The XHCI controller will not be functional, but that's both expected and not a
deal-breaker for getting the machine into a usable state.

[0] https://cgit.freebsd.org/src/tree/sys/arm64/arm64/busdma_bounce.c#n572

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