lguest: documentation V: Host
Documentation: The Host
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/drivers/lguest/lg.h b/drivers/lguest/lg.h
index 3b9dc12..269116e 100644
--- a/drivers/lguest/lg.h
+++ b/drivers/lguest/lg.h
@@ -58,9 +58,18 @@
u8 interrupt; /* 0 when not registered */
};
-/* We have separate types for the guest's ptes & pgds and the shadow ptes &
- * pgds. Since this host might use three-level pagetables and the guest and
- * shadow pagetables don't, we can't use the normal pte_t/pgd_t. */
+/*H:310 The page-table code owes a great debt of gratitude to Andi Kleen. He
+ * reviewed the original code which used "u32" for all page table entries, and
+ * insisted that it would be far clearer with explicit typing. I thought it
+ * was overkill, but he was right: it is much clearer than it was before.
+ *
+ * We have separate types for the Guest's ptes & pgds and the shadow ptes &
+ * pgds. There's already a Linux type for these (pte_t and pgd_t) but they
+ * change depending on kernel config options (PAE). */
+
+/* Each entry is identical: lower 12 bits of flags and upper 20 bits for the
+ * "page frame number" (0 == first physical page, etc). They are different
+ * types so the compiler will warn us if we mix them improperly. */
typedef union {
struct { unsigned flags:12, pfn:20; };
struct { unsigned long val; } raw;
@@ -77,8 +86,12 @@
struct { unsigned flags:12, pfn:20; };
struct { unsigned long val; } raw;
} gpte_t;
+
+/* We have two convenient macros to convert a "raw" value as handed to us by
+ * the Guest into the correct Guest PGD or PTE type. */
#define mkgpte(_val) ((gpte_t){.raw.val = _val})
#define mkgpgd(_val) ((gpgd_t){.raw.val = _val})
+/*:*/
struct pgdir
{