Re: [PATCH v5 1/6] Revert "xen/x86: bzImage parse kernel_alignment"

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Mar 27, 2024 at 08:22:41AM +0100, Jan Beulich wrote:
> On 26.03.2024 22:38, Jason Andryuk wrote:
> > A new ELF note will specify the alignment for a relocatable PVH kernel.
> > ELF notes are suitable for vmlinux and other ELF files, so this
> > Linux-specific bzImage parsing in unnecessary.
> > 
> > This reverts commit c44cac229067faeec8f49247d1cf281723ac2d40.
> > 
> > Signed-off-by: Jason Andryuk <jason.andryuk@xxxxxxx>
> 
> Since you keep re-sending this: In private discussion Roger has indicated
> that, like me, he too would prefer falling back to the ELF data, before
> falling back to the arch default (Roger, please correct me if I got it
> wrong). That would make it necessary that the change you're proposing to
> revert here is actually kept.

Sorry, was meaning to reply yesterday but Jason is very fast at
sending new version so I'm always one version behind.

IMO the order: ELF note, PHDR alignment, arch default should be the
preferred one.

> Or wait - what you're reverting is taking the alignment out of the
> bzImage header. I don't expect the BSDs to use that protocol; aiui that's
> entirely Linux-specific.

Yeah, I don't have strong opinions in keeping this, we already do
bzImage parsing, so we might as well attempt to fetch the alignment
from there if correct:

ELF note, bzImage kernel_alignment, ELF PHDR alignment, arch default

> I further meanwhile realized that consulting the ELF phdrs may also be
> ambiguous, as there may be more than one. I guess it would need to be the
> maximum of all of them then.

My suggestion (not sure if I mentioned this before) was to use the
alignment of the first LOAD PHDR, which is the one that defines the
value of the dest_base field used as the image load start address.

Using the maximum of all load PHDRs might be safer.

Thanks, Roger.