hw/arm_boot: Align device tree to 4KB boundary, not page

Align the device tree blob to a 4KB boundary, not to QEMU's
idea of a page boundary -- the latter is the smallest possible
page size for the architecture, which on ARM is 1KB.
The documentation for Linux does not impose separation
or alignment requirements on the device tree blob, but
in practice some kernels will happily trash the entire
page the initrd ends in after they have finished uncompressing
the initrd. So 4KB-align the DTB to ensure it does not get
trampled by these kernels.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>

hw/arm_boot.c

@@ -441,9 +441,12 @@ void arm_load_kernel(ARMCPU *cpu, struct arm_boot_info *info)
          * we point to the kernel args.
          */
         if (info->dtb_filename) {
-            /* Place the DTB after the initrd in memory */
-            hwaddr dtb_start = TARGET_PAGE_ALIGN(info->initrd_start +
-                                                 initrd_size);
+            /* Place the DTB after the initrd in memory. Note that some
+             * kernels will trash anything in the 4K page the initrd
+             * ends in, so make sure the DTB isn't caught up in that.
+             */
+            hwaddr dtb_start = QEMU_ALIGN_UP(info->initrd_start + initrd_size,
+                                             4096);
             if (load_dtb(dtb_start, info)) {
                 exit(1);
             }