bhyve(8): Add bootrom allocation abstraction

[cem]

Summary

To allow more general use of the bootrom region, separate initialization from
allocation, and allocation from loading a file.

The bootrom segment is the high 16MB of the low 4GB region.

Each allocation in the segment creates a new mapping with specified protection.
By default, allocation begins at the low end of the range. However, the
BOOTROM_ALLOC_TOP flag is provided to locate a provided bootrom in the high
region it is expected to be in.

The existing ROM-file loading code is refactored to use the new interface.

[grehan]

usr.sbin/bhyve/bootrom.c

59 ↗

(On Diff #70569)

The exception to first-come is the actual boot rom, which must go into the area containing the power-on reset vector. Maybe a bool parameter to request this type of allocation ? Fine for now.

[cem]

usr.sbin/bhyve/bootrom.c

59 ↗

(On Diff #70569)

I think I’ve placed it in the identical region as it was before this change. I agree a flag to allocate top vs bottom would help guarantee this regardless of the order of requests.

[cem]

• Add a flag to allocate top, and default to bottom
• Use top flag in actual bootrom logic
• Fix the allocation logic to actually update the free region, which was previously absent. Oops.