Add linear_phys_to_virt and vice versa.

However, since not all platforms will allow the entire physical address
space to be simultaneously mapped to part of the virtual address space,
we may still require some dynamic mapping.

src/arch/x86_64/paging/mod.rs

@@ -335,6 +335,14 @@ impl InactivePageTable {
     }
 }
 
+pub fn linear_phys_to_virt(physical: PhysicalAddress) -> Option<VirtualAddress> {
+    physical.data().checked_add(crate::KERNEL_OFFSET).map(VirtualAddress::new)
+}
+pub fn linear_virt_to_phys(virt: VirtualAddress) -> Option<PhysicalAddress> {
+    virt.data().checked_sub(crate::KERNEL_OFFSET).map(PhysicalAddress::new)
+}
+
+
 /// Page
 #[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
 pub struct Page {