lguest: use a special 1:1 linear pagetable mode until first switch.

The Host used to create some page tables for the Guest to use at the
top of Guest memory; it would then tell the Guest where this was.  In
particular, it created linear mappings for 0 and 0xC0000000 addresses
because lguest used to switch to its real page tables quite late in
boot.

However, since d50d8fe19 Linux initialized boot page tables in
head_32.S even before the "are we lguest?" boot jump.  So, now we can
simplify things: the Host pagetable code assumes 1:1 linear mapping
until it first calls the LHCALL_NEW_PGTABLE hypercall, which we now do
before we reach C code.

This also means that the Host doesn't need to know anything about the
Guest's PAGE_OFFSET.  (Non-Linux guests might not even have such a
thing).

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

1 files changed, 5 insertions, 6 deletions

diff --git a/arch/x86/lguest/boot.c b/arch/x86/lguest/boot.c
index db832fd65ecb..719a32c60516 100644
--- a/arch/x86/lguest/boot.c
+++ b/arch/x86/lguest/boot.c
@@ -520,17 +520,16 @@ static unsigned long lguest_read_cr2(void)
 
 /* See lguest_set_pte() below. */
 static bool cr3_changed = false;
+static unsigned long current_cr3;
 
 /*
  * cr3 is the current toplevel pagetable page: the principle is the same as
- * cr0.  Keep a local copy, and tell the Host when it changes.  The only
- * difference is that our local copy is in lguest_data because the Host needs
- * to set it upon our initial hypercall.
+ * cr0.  Keep a local copy, and tell the Host when it changes.
  */
 static void lguest_write_cr3(unsigned long cr3)
 {
-        lguest_data.pgdir = cr3;
         lazy_hcall1(LHCALL_NEW_PGTABLE, cr3);
+        current_cr3 = cr3;
 
         /* These two page tables are simple, linear, and used during boot */
         if (cr3 != __pa(swapper_pg_dir) && cr3 != __pa(initial_page_table))
@@ -539,7 +538,7 @@ static void lguest_write_cr3(unsigned long cr3)
 
 static unsigned long lguest_read_cr3(void)
 {
-        return lguest_data.pgdir;
+        return current_cr3;
 }
 
 /* cr4 is used to enable and disable PGE, but we don't care. */
@@ -758,7 +757,7 @@ static void lguest_pmd_clear(pmd_t *pmdp)
 static void lguest_flush_tlb_single(unsigned long addr)
 {
         /* Simply set it to zero: if it was not, it will fault back in. */
-        lazy_hcall3(LHCALL_SET_PTE, lguest_data.pgdir, addr, 0);
+        lazy_hcall3(LHCALL_SET_PTE, current_cr3, addr, 0);
 }
 
 /*