Re: [PATCH 2/7] multiboot2: Allow 64-bit entry tags

[Ross Lagerwall]

On Tue, Mar 19, 2024 at 10:07 AM Roger Pau Monné <roger.pau@citrix.com> wrote:
>
> On Wed, Mar 13, 2024 at 03:07:43PM +0000, Ross Lagerwall wrote:
> > Binaries may be built with entry points above 4G. While bootloaders may
> > relocate them below 4G, it should be possible for the binary to specify
> > those entry points. Therefore, extend the multiboot2 protocol such that
> > 64 bit addresses are allowed for entry points. The extension is done in
> > a backwards-compatible way.
> >
> > Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
> > ---
> >  doc/multiboot.texi | 32 +++++++++++++++++++-------------
> >  doc/multiboot2.h   |  6 +++++-
> >  2 files changed, 24 insertions(+), 14 deletions(-)
> >
> > diff --git a/doc/multiboot.texi b/doc/multiboot.texi
> > index d12719c744eb..049afab53c1f 100644
> > --- a/doc/multiboot.texi
> > +++ b/doc/multiboot.texi
> > @@ -522,12 +522,12 @@ header.
> >
> >  @example
> >  @group
> > -        +-------------------+
> > -u16     | type = 3          |
> > -u16     | flags             |
> > -u32     | size              |
> > -u32     | entry_addr        |
> > -        +-------------------+
> > +          +-------------------+
> > +u16       | type = 3          |
> > +u16       | flags             |
> > +u32       | size              |
> > +u32 / u64 | entry_addr        |
> > +          +-------------------+
>
> I might be confused, but this entry point is used in 32bit protected
> mode, and hence a 64bit value is simply impossible to use according to
> the protocol in "3.3 I386 machine state".
>
> Unless that section is expanded to describe other protocols that use
> the entry address in a way where 64bits could be meaningful it seems
> pointless to expand the field.

I changed this because the same binary is being used for both BIOS boot
and UEFI boot, therefore it may have a base address above 4 GiB.
Despite that, it is expected that GRUB would relocate the binary below
4 GiB so BIOS boot would still work.

However, on reflection this is kind of nasty. I've managed to build Xen
in such a way that this is no longer needed so I can drop this change
from the next version of this series.

Ross