memory: use signed arithmetic

When trying to map an alias of a ram region, where the alias starts at
address A and we map it into address B, and A > B, we had an arithmetic
underflow.  Because we use unsigned arithmetic, the underflow converted
into a large number which failed addrrange_intersects() tests.

The concrete example which triggered this was cirrus vga mapping
the framebuffer at offsets 0xc0000-0xc7fff (relative to the start of
the framebuffer) into offsets 0xa0000 (relative to system addres space
start).

With our favorite analogy of a windowing system, this is equivalent to
dragging a subwindow off the left edge of the screen, and failing to clip
it into its parent window which is on screen.

Fix by switching to signed arithmetic.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/exec.c b/exec.c
index 476b507e5e..751fd8967b 100644
--- a/exec.c
+++ b/exec.c
@@ -3818,7 +3818,7 @@ static void io_mem_init(void)
 static void memory_map_init(void)
 {
     system_memory = qemu_malloc(sizeof(*system_memory));
-    memory_region_init(system_memory, "system", UINT64_MAX);
+    memory_region_init(system_memory, "system", INT64_MAX);
     set_system_memory_map(system_memory);
 }