Re: [Qemu-devel] Support for multiboot images in elf64 (EM_X86_64) format

[Stefan Hajnoczi <stefanha@gmail.com>]

On Sat, Feb 4, 2012 at 5:38 PM, Goswin von Brederlow <goswin-v-b@web.de> wrote:
> Description: Allow 64bit elf binaries in multiboot format
>  This patch allows 64bit elf files with multiboot header to be loaded.
>  The entry point will still be called in 32bit mode and the kernel
>  must switch to 64bit mode on its own. The image and all modules must
>  also be located in the lower 2GB of ram. All the restrictions of a
>  32bit image still apply.
> Author: Goswin von Brederlow <goswin-v-b@web.de>
> Last-Updated: 2011-04-08
> ---

The multiboot specification is 32-bit only.  This patch enables a
non-standard 64-bit version of multiboot.  Have you checked whether
GRUB or other multiboot loaders have equivalent functionality?  Have
you contacted the multiboot specification authors?

> --- qemu-kvm-0.14.0+dfsg.orig/hw/multiboot.c
> +++ qemu-kvm-0.14.0+dfsg/hw/multiboot.c
> @@ -173,8 +173,7 @@ int load_multiboot(void *fw_cfg,
>         fclose(f);
>
>         if (((struct elf64_hdr*)header)->e_machine == EM_X86_64) {
> -            fprintf(stderr, "Cannot load x86-64 image, give a 32bit one.\n");
> -            exit(1);
> +           mb_debug("qemu: 64bit elf, I hope you know what you are doing\n");

This is silent by default, but given the nature of 64-bit multiboot
support I think this warning should be on by default.  Anyone using
this really needs to know what they are doing and QEMU should not
silently do weird things.

Stefan