Re: [PATCH 0/5] Boot protocol changes

[H. Peter Anvin]

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

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