drop ", but only"

s/yields/allocates

memory is used redundantly. I would say 'if memory with special constraints such as attributes, alignment, or address ranges must be used..."

I think this could be phrased better as a constraint than a statement of fact.

"Objects must be physically contiguous"

Although that still seems to imply that somehow multiple objects are contiguous. We really want to express that a single object will be.

There really shouldn't be any boot time contig allocations but I suppose it doesn't hurt to handle it.

Okay, and then drop the VM_MEMATTR_DEFAULT example?

This is gratuitous, but maybe related enough to sneak in here.

Also gratuitous, but related.

Yes... I thought to handle it since it's easy and I think it works fine with startup_alloc. If you prefer, I can add it to the KASSERT about early boot PCPU alloc instead.

Yeah, I'm having trouble phrasing this concisely with the right sense. I think your suggestion is okay. Here's another idea: "Each object must be internally physically contiguous."

Your proposed phrasing, specifically "Each object", is less likely to be misinterpreted.

No description that fits in one line will be unambiguous, If you are willing to use two lines, I would suggest: "Physical memory underlying an object must be contiguous"

I would suggest: "The physical memory underlying each item in this zone must be contiguous. In particular, if an item spans a page boundary, then the underlying physical pages must be contiguous."

Thanks, I'll use that.