Introduce guest mem offset, static link example launcher

In order to avoid problematic special linking of the Launcher, we give
the Host an offset: this means we can use any memory region in the
Launcher as Guest memory rather than insisting on mmap() at 0.

The result is quite pleasing: a number of casts are replaced with
simple additions.

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

1 files changed, 9 insertions, 6 deletions

diff --git a/drivers/lguest/hypercalls.c b/drivers/lguest/hypercalls.c
index 5ecd60b54201..02e67b49ea4f 100644
--- a/drivers/lguest/hypercalls.c
+++ b/drivers/lguest/hypercalls.c
@@ -205,16 +205,19 @@ static void initialize(struct lguest *lg)
                 tsc_speed = 0;
 
         /* The pointer to the Guest's "struct lguest_data" is the only
-         * argument. */
-        lg->lguest_data = (struct lguest_data __user *)lg->regs->edx;
-        /* If we check the address they gave is OK now, we can simply
-         * copy_to_user/from_user from now on rather than using lgread/lgwrite.
-         * I put this in to show that I'm not immune to writing stupid
-         * optimizations. */
+         * argument.  We check that address now. */
         if (!lguest_address_ok(lg, lg->regs->edx, sizeof(*lg->lguest_data))) {
                 kill_guest(lg, "bad guest page %p", lg->lguest_data);
                 return;
         }
+
+        /* Having checked it, we simply set lg->lguest_data to point straight
+         * into the Launcher's memory at the right place and then use
+         * copy_to_user/from_user from now on, instead of lgread/write.  I put
+         * this in to show that I'm not immune to writing stupid
+         * optimizations. */
+        lg->lguest_data = lg->mem_base + lg->regs->edx;
+
         /* The Guest tells us where we're not to deliver interrupts by putting
          * the range of addresses into "struct lguest_data". */
         if (get_user(lg->noirq_start, &lg->lguest_data->noirq_start)