linux-user: reserve 4GB of vmem for 32-on-64

When running 32-on-64 bit guests, we should always reserve as much
virtual memory as we possibly can for the guest process, so it can
never overlap with QEMU address space.

Fortunately we already have the infrastructure for that. All that's
missing is some sane default value to also make use of it!
Signed-off-by: NAlexander Graf <agraf@suse.de>
Signed-off-by: NRiku Voipio <riku.voipio@linaro.org>

linux-user/main.c

@@ -48,8 +48,19 @@ unsigned long mmap_min_addr;
 #if defined(CONFIG_USE_GUEST_BASE)
 unsigned long guest_base;
 int have_guest_base;
+#if (TARGET_LONG_BITS == 32) && (HOST_LONG_BITS == 64)
+/*
+ * When running 32-on-64 we should make sure we can fit all of the possible
+ * guest address space into a contiguous chunk of virtual host memory.
+ *
+ * This way we will never overlap with our own libraries or binaries or stack
+ * or anything else that QEMU maps.
+ */
+unsigned long reserved_va = 0xf7000000;
+#else
 unsigned long reserved_va;
 #endif
+#endif
 
 static void usage(void);