Re: [RFC PATCH] x86/boot: make ELF kernel multiboot-able

[Paolo Bonzini]

On 15/02/2017 15:41, Chao Peng wrote:
> Multiboot specification (http://git.savannah.gnu.org/cgit/grub.git/tree/doc/multiboot.texi?h=multiboot2)
> is an open standard that provides kernels with a uniform way to be booted
> by multiboot-compliant bootloaders (like grub).
> 
> This patch is trying to make Linux ELF kernel image to be a
> multiboot-compliant OS so that it can be loaded by a multiboot-comliant
> bootloader. The benefit is eliminating the maintainance for realmode and
> decompression code and especially when the kernel is loaded in a virtual
> machine, the reducing for these code can greatly cuts down the boot time.
> 
> However, the current version of multiboot spec doesn't support 64 bit
> well so for 64 bit kernel we need stub code to jump from 32 bit code to
> 64 bit code. Besides, there are still some other issues:
>   1). '-z max-page-size=0x1000' is used so the text segment start is in
>   multiboot header search scope because GNU LD has default page size of
>   0x00200000 for ELF64, which will fail multiboot test.
> 
>   2). The bootloader like grub has support for ELF kernel (even for ELF64)
>   which makes the patch easier. However, the current grub implementaion
>   thinks the entry address should be a VA. E.g. for 64 bit kernel, the entry
>   address (0x1000000) is actually phiscial address, grub refuses to load it
>   by saying: 'entry point isn't in a segment'.

For kvm-unit-tests, we do "objcopy -O elf32-i386 dest.32bit dest.64bit"
and pass the resulting 32bit ELF file to grub.

Out of curiosity, what happens if you pass the resulting multiboot file
to QEMU's -kernel option?

Thanks,

Paolo

> This patch is sent out as RFC in case you have some ideas.
> 
> Signed-off-by: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>