lguest: map Switcher below fixmap.

Now we've adjusted all the code, we can simply set switcher_addr to
wherever it needs to go below the fixmaps, rather than asserting that
it should be so.

With large NR_CPUS and PAE, people were hitting the "mapping switcher
would thwack fixmap" message.
Reported-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

arch/x86/include/asm/lguest.h

@@ -14,12 +14,6 @@
 /* Page for Switcher text itself, then two pages per cpu */
 #define TOTAL_SWITCHER_PAGES (1 + 2 * nr_cpu_ids)
 
-/* We map at -4M (-2M for PAE) for ease of mapping (one PTE page). */
-#ifdef CONFIG_X86_PAE
-#define SWITCHER_ADDR 0xFFE00000
-#else
-#define SWITCHER_ADDR 0xFFC00000
-#endif
 /* Where we map the Switcher, in both Host and Guest. */
 extern unsigned long switcher_addr;
 

drivers/lguest/core.c

@@ -83,18 +83,13 @@ static __init int map_switcher(void)
 		}
 	}
 
-	switcher_addr = SWITCHER_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.
+	 * We place the Switcher underneath the fixmap area, which is the
+	 * highest virtual address we can get.  This is important, since we
+	 * tell the Guest it can't access this memory, so we want its ceiling
+	 * as high as possible.
 	 */
-	if (switcher_addr + (TOTAL_SWITCHER_PAGES+1)*PAGE_SIZE > FIXADDR_START){
-		err = -ENOMEM;
-		printk("lguest: mapping switcher would thwack fixmap\n");
-		goto free_pages;
-	}
+	switcher_addr = FIXADDR_START - (TOTAL_SWITCHER_PAGES+1)*PAGE_SIZE;
 
 	/*
 	 * Now we reserve the "virtual memory area" we want.  We might