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Date: Mon, 6 Feb 2012 12:38:14 +0000
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From: Stefan Hajnoczi <stefanha@gmail.com>
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Subject: Re: [Qemu-devel] Support for multiboot images in elf64 (EM_X86_64)
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To: Goswin von Brederlow <goswin-v-b@web.de>
Cc: qemu-devel@nongnu.org

On Sat, Feb 4, 2012 at 5:38 PM, Goswin von Brederlow <goswin-v-b@web.de> wr=
ote:
> Description: Allow 64bit elf binaries in multiboot format
> =A0This patch allows 64bit elf files with multiboot header to be loaded.
> =A0The entry point will still be called in 32bit mode and the kernel
> =A0must switch to 64bit mode on its own. The image and all modules must
> =A0also be located in the lower 2GB of ram. All the restrictions of a
> =A032bit 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,
> =A0 =A0 =A0 =A0 fclose(f);
>
> =A0 =A0 =A0 =A0 if (((struct elf64_hdr*)header)->e_machine =3D=3D EM_X86_=
64) {
> - =A0 =A0 =A0 =A0 =A0 =A0fprintf(stderr, "Cannot load x86-64 image, give =
a 32bit one.\n");
> - =A0 =A0 =A0 =A0 =A0 =A0exit(1);
> + =A0 =A0 =A0 =A0 =A0 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