Document physical memory mapping size

api/src/config.rs

@@ -386,10 +386,16 @@ pub struct Mappings {
     pub boot_info: Mapping,
     /// Specifies the mapping of the frame buffer memory region.
     pub framebuffer: Mapping,
-    /// The bootloader supports to map the whole physical memory into the virtual address
+    /// The bootloader supports mapping the whole physical memory into the virtual address
     /// space at some offset. This is useful for accessing and modifying the page tables set
     /// up by the bootloader.
     ///
+    /// This mapping will go from physical address `0x0` to whichever is larger:
+    /// - The end of the last region in the BIOS/UEFI memory map
+    /// - The address `0x1_0000_0000` (such that at least 4 GiB of physical memory are always mapped).
+    ///   This is to ensure that useful MMIO regions (local APIC, I/O APIC, PCI bars) are
+    ///   accessible to the kernel even if less physical memory than that is on the system.
+    ///
     /// Defaults to `None`, i.e. no mapping of the physical memory.
     pub physical_memory: Option<Mapping>,
     /// As an alternative to mapping the whole physical memory (see [`Self::physical_memory`]),