lguest: fix __get_vm_area usage.

Robert Bragg's 5dc331852848a38ca00a2817e5b98a1d0561b116 tightened
(ie. fixed) the checking in __get_vm_area, and it broke lguest.

lguest should pass the exact "end" it wants, not some random constant
(it was possible previously that it would actually get an address
different from SWITCHER_ADDR).

Also, Fabio Checconi pointed out that we should make sure we're not
hitting the fixmap area.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: Robert Bragg <robert@sixbynine.org>

drivers/lguest/core.c

@@ -69,11 +69,22 @@ static __init int map_switcher(void)
 		switcher_page[i] = virt_to_page(addr);
 	}
 
+	/* First we check that the Switcher won't overlap the fixmap area at
+	 * the top of memory.  It's currently nowhere near, but it could have
+	 * very strange effects if it ever happened. */
+	if (SWITCHER_ADDR + (TOTAL_SWITCHER_PAGES+1)*PAGE_SIZE > FIXADDR_START){
+		err = -ENOMEM;
+		printk("lguest: mapping switcher would thwack fixmap\n");
+		goto free_pages;
+	}
+
 	/* Now we reserve the "virtual memory area" we want: 0xFFC00000
 	 * (SWITCHER_ADDR).  We might not get it in theory, but in practice
-	 * it's worked so far. */
+	 * it's worked so far.  The end address needs +1 because __get_vm_area
+	 * allocates an extra guard page, so we need space for that. */
 	switcher_vma = __get_vm_area(TOTAL_SWITCHER_PAGES * PAGE_SIZE,
-				       VM_ALLOC, SWITCHER_ADDR, VMALLOC_END);
+				     VM_ALLOC, SWITCHER_ADDR, SWITCHER_ADDR
+				     + (TOTAL_SWITCHER_PAGES+1) * PAGE_SIZE);
 	if (!switcher_vma) {
 		err = -ENOMEM;
 		printk("lguest: could not map switcher pages high\n");