linux-user: fix mmap_find_vma_reserved()

The value given by mmap_find_vma_reserved() is used with mmap(),
so it is needed to be aligned with the host page size.

Since commit 18e80c55bb, reserved_va is only aligned to TARGET_PAGE_SIZE,
and it works well if this size is greater or equal to the host page size.

But ppc64 hosts have 64kB page size and when we start a 4kiB page size
guest (like i386), it fails when it tries to mmap the stack:

    mmap stack: Invalid argument

Fixes: 18e80c55bb (linux-user: Tidy and enforce reserved_va initialization)
Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20180714193553.30846-1-laurent@vivier.eu>

linux-user/main.c

@@ -78,14 +78,7 @@ int have_guest_base;
 # endif
 #endif
 
-/* That said, reserving *too* much vm space via mmap can run into problems
-   with rlimits, oom due to page table creation, etc.  We will still try it,
-   if directed by the command-line option, but not by default.  */
-#if HOST_LONG_BITS == 64 && TARGET_VIRT_ADDR_SPACE_BITS <= 32
-unsigned long reserved_va = MAX_RESERVED_VA;
-#else
 unsigned long reserved_va;
-#endif
 
 static void usage(int exitcode);
 
@@ -672,6 +665,18 @@ int main(int argc, char **argv, char **envp)
     /* init tcg before creating CPUs and to get qemu_host_page_size */
     tcg_exec_init(0);
 
+    /* Reserving *too* much vm space via mmap can run into problems
+       with rlimits, oom due to page table creation, etc.  We will still try it,
+       if directed by the command-line option, but not by default.  */
+    if (HOST_LONG_BITS == 64 &&
+        TARGET_VIRT_ADDR_SPACE_BITS <= 32 &&
+        reserved_va == 0) {
+        /* reserved_va must be aligned with the host page size
+         * as it is used with mmap()
+         */
+        reserved_va = MAX_RESERVED_VA & qemu_host_page_mask;
+    }
+
     cpu = cpu_create(cpu_type);
     env = cpu->env_ptr;
     cpu_reset(cpu);