Avoid mapping the same MMIO pages multiple times.

[todo]

Currently, if we need to map distinct MMIO regions sharing the same page, we do multiple mapping. This is wasteful of address space, which is a relatively scarce resource.
It might be a good idea to make an MMIO manager that hands out references to the same mapping (with different offsets) when a single page is shared.

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SunriseOS/kernel/src/devices/ioapic.rs

Lines 227 to 237 in 8012637

	// TODO: Avoid mapping the same MMIO pages multiple times.
	// BODY: Currently, if we need to map distinct MMIO regions sharing the
	// BODY: same page, we do multiple mapping. This is wasteful of address
	// BODY: space, which is a relatively scarce resource.
	// BODY:
	// BODY: It might be a good idea to make an MMIO manager that hands out
	// BODY: references to the same mapping (with different offsets) when
	// BODY: a single page is shared.
	if address.floor() != (address + 8).floor() {
	panic!("Weird MMIO.")
	}

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This issue was generated by todo based on a TODO comment in 8012637 when #304 was merged. cc @roblabla.