Re: [PATCH 0/5] Boot protocol changes

H. Peter Anvin
2007-10-03T00:44:09Z

Jeremy Fitzhardinge wrote:
> H. Peter Anvin wrote:
>>> This series looks like a good start for Xen, but we still need to work
>>> out where to stash the metadata which normally lives in ELF notes.
>>> Using ELF is convenient for Xen because it lets a large chunk of domain
>>> builder code be reused; on the other hand, loading a plain bzImage is
>>> pretty simple, so maybe it isn't such a big deal.
>>>
>>> HPA, Eric: if we don't go the "embed ELF" path, where's a good
>>> backwards-compatible place to stash the note data? If we do go with
>>> "embed ELF", how should we go about doing it? Arrange to put the ELF
>>> headers before the 1M mark?
>>>
>> This sounds like another good reason to do the ELF image as the
>> postcompression image. The interface to the embedded compression
>> routine is then unchanged, and we get the "full vmlinux" with any
>> notes that belongs there.
>>
>> I'll try to get an implementation of that done -- it really shouldn't
>> be very hard.
>
> Please explain what you're proposing again, because my memory of your
> plan from last time wouldn't help in this case. Are you proposing that
> the bzImage contains compressed data that its expecting the bootloader
> to decompress? Won't that completely break backwards compatibility? If
> we don't care about backwards compatibility with old bootloaders, then
> it doesn't matter what we do one way or the other.
>

No, not at all.

I'm proposing that the existing bzImage format be retained, but that the
payload of the decompressor (already a gzip file) simply be vmlinux.gz
-- i.e. a gzip compressed ELF file, notes and all. A pointer in the
header will point to the offset of the payload (this is new, obviously.)

The decompression stub is adjusted to expect an ELF image, instead of a
raw binary.

Existing bootloaders (16- or 32-bit) simply load the bzImage the way
they do now; new bootloaders have the option of accessing the vmlinux.gz
directly if they either want to load it themselves or want to examine
the notes.

-hpa