rust: page: Add physical address conversion functions

Add methods to allow code using the Page type to obtain the physical
address of a page, convert to and from an (owned) physical address, and
borrow a Page from a physical address. Most of these operations are, as
you might expect, unsafe.

These primitives are useful to implement page table structures in Rust,
and to implement arbitrary physical memory access (as needed to walk
arbitrary page tables and dereference through them). These mechanisms
are, of course, fraught with danger, and are only expected to be used
for core memory management code (in e.g. drivers with their own device
page table implementations) and for debug features such as crash dumps
of device memory.

Signed-off-by: Asahi Lina <lina@asahilina.net>

Author: hoshinolina

rust/helpers/page.c

@@ -1,5 +1,6 @@
 // SPDX-License-Identifier: GPL-2.0
 
+#include <asm/io.h>
 #include <linux/gfp.h>
 #include <linux/highmem.h>
 
@@ -17,3 +18,28 @@ void rust_helper_kunmap_local(const void *addr)
 {
 	kunmap_local(addr);
 }
+
+struct page *rust_helper_phys_to_page(phys_addr_t phys)
+{
+	return phys_to_page(phys);
+}
+
+phys_addr_t rust_helper_page_to_phys(struct page *page)
+{
+	return page_to_phys(page);
+}
+
+unsigned long rust_helper_phys_to_pfn(phys_addr_t phys)
+{
+	return __phys_to_pfn(phys);
+}
+
+struct page *rust_helper_pfn_to_page(unsigned long pfn)
+{
+	return pfn_to_page(pfn);
+}
+
+bool rust_helper_pfn_is_valid(unsigned long pfn)
+{
+	return pfn_valid(pfn);
+}

rust/kernel/page.rs

@@ -3,13 +3,15 @@
 //! Kernel page allocation and management.
 
 use crate::{
+    addr::*,
     alloc::{AllocError, Flags},
     bindings,
     error::code::*,
     error::Result,
     types::{Opaque, Ownable, Owned},
     uaccess::UserSliceReader,
 };
+use core::mem::ManuallyDrop;
 use core::ptr::{self, NonNull};
 
 /// A bitwise shift for the page size.
@@ -249,6 +251,55 @@ impl Page {
             reader.read_raw(unsafe { core::slice::from_raw_parts_mut(dst.cast(), len) })
         })
     }
+
+    /// Returns the physical address of this page.
+    pub fn phys(&self) -> PhysicalAddr {
+        // SAFETY: `page` is valid due to the type invariants on `Page`.
+        unsafe { bindings::page_to_phys(self.as_ptr()) }
+    }
+
+    /// Converts a Rust-owned Page into its physical address.
+    /// The caller is responsible for calling `from_phys()` to avoid
+    /// leaking memory.
+    pub fn into_phys(this: Owned<Self>) -> PhysicalAddr {
+        ManuallyDrop::new(this).phys()
+    }
+
+    /// Converts a physical address to a Rust-owned Page.
+    ///
+    /// SAFETY:
+    /// The caller must ensure that the physical address was previously returned
+    /// by a call to `Page::into_phys()`, and that the physical address is no
+    /// longer used after this call, nor is `from_phys()` called again on it.
+    pub unsafe fn from_phys(phys: PhysicalAddr) -> Owned<Self> {
+        // SAFETY: By the safety requirements, the physical address must be valid and
+        // have come from `into_phys()`, so phys_to_page() cannot fail and
+        // must return the original struct page pointer.
+        unsafe { Owned::from_raw(NonNull::new_unchecked(bindings::phys_to_page(phys)).cast()) }
+    }
+
+    /// Borrows a Page from a physical address, without taking over ownership.
+    ///
+    /// If the physical address does not have a `struct page` entry, returns
+    /// None.
+    ///
+    /// SAFETY:
+    /// The caller must ensure that the physical address, if it is backed by a
+    /// `struct page`, remains available for the duration of the borrowed
+    /// lifetime.
+    pub unsafe fn borrow_phys(phys: &PhysicalAddr) -> Option<&Self> {
+        // SAFETY: This is always safe, as it is just arithmetic
+        let pfn = unsafe { bindings::phys_to_pfn(*phys) };
+        // SAFETY: This function is safe to call with any pfn
+        if !unsafe { bindings::pfn_is_valid(pfn) } {
+            None
+        } else {
+            // SAFETY: We have just checked that the pfn is valid above, so it must
+            // have a corresponding struct page. By the safety requirements, we can
+            // return a borrowed reference to it.
+            Some(unsafe { &*(bindings::pfn_to_page(pfn) as *mut Self as *const Self) })
+        }
+    }
 }
 
 // SAFETY: See below.