[PATCH] arm64: booting: Require placement within 48-bit addressable memory

[Ard Biesheuvel]

On Mon, 14 Nov 2022 at 13:10, Mark Rutland <mark.rutland at arm.com> wrote:
>
> On Mon, Nov 14, 2022 at 12:58:10PM +0100, Ard Biesheuvel wrote:
> > Some configurations (i.e., 64k + LVA/LPA) can tolerate a physical
> > placement of the kernel image outside of the 48-bit addressable region,
> > but given that the loader has no way of knowing whether or not the image
> > in question supports LVA/LPA, it currently has no choice but to place it
> > below the 48-bit mark.
> >
> > Once we add support for LPA2, which allows 52-bit physical and virtual
> > addressing when using 4k or 16k pages, but in way that relies on
> > increasing the number of paging levels, there will be more variety in
> > the configurations that may or may not support this.
> >
> > So repaint bit #3 in the Image header as 'must be placed within 48-bit
> > addressable memory', as this is the current de facto meaning, and add
> > bits that we may set later to allow placement outside of that.
>
> Could we just clean up the definition of bit 3 for now, then add the other bits
> when we actually set them?
>

Sure

> >
> > Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
> > ---
> >  Documentation/arm64/booting.rst | 9 +++++++--
> >  1 file changed, 7 insertions(+), 2 deletions(-)
> >
> > diff --git a/Documentation/arm64/booting.rst b/Documentation/arm64/booting.rst
> > index 8c324ad638de2b27..9919e1a107add655 100644
> > --- a/Documentation/arm64/booting.rst
> > +++ b/Documentation/arm64/booting.rst
> > @@ -114,16 +114,21 @@ Header notes:
> >                       * 1 - 4K
> >                       * 2 - 16K
> >                       * 3 - 64K
> > -  Bit 3              Kernel physical placement
> > +  Bit 3-4    Kernel physical placement
> >
> >                       0
> >                         2MB aligned base should be as close as possible
> >                         to the base of DRAM, since memory below it is not
> >                         accessible via the linear mapping
> >                       1
> > +                       2MB aligned base such that base plus image_size is
> > +                       entirely within the 48-bit addressable range
>
> This seems to assume text_offset is zero (which is the case today, but this is
> the first part of booting.txt that assumes that); as a matter of policy are we
> sure that we'll never use a non-zero offset again? If so, we probably want to
> mention that in the "header notes" section, or come up with some wording to
> include it here.
>

How about:

2MB aligned base such that all image_size bytes
counted from the start of the image are within
the 48-bit addressable range

?

> Otherwise, this looks good; I'm certainly in favour of explicitly mentioning
> the 48-bit restriction.
>
> Thanks,
> Mark.
>