Re: [PATCH] hw/arm/boot: set initrd parameters to 64bit in fdt

[Peter Maydell]

On Tue, 8 Nov 2022 at 12:52, Schspa Shi <schspa@gmail.com> wrote:
> Alex Bennée <alex.bennee@linaro.org> writes:
> > There is a whole comment in boot.c talking about keeping initrd within
> > lowmem:
> >
> >     /*
> >      * We want to put the initrd far enough into RAM that when the
> >      * kernel is uncompressed it will not clobber the initrd. However
> >      * on boards without much RAM we must ensure that we still leave
> >      * enough room for a decent sized initrd, and on boards with large
> >      * amounts of RAM we must avoid the initrd being so far up in RAM
> >      * that it is outside lowmem and inaccessible to the kernel.
> >      * So for boards with less  than 256MB of RAM we put the initrd
> >      * halfway into RAM, and for boards with 256MB of RAM or more we put
> >      * the initrd at 128MB.
> >      * We also refuse to put the initrd somewhere that will definitely
> >      * overlay the kernel we just loaded, though for kernel formats which
> >      * don't tell us their exact size (eg self-decompressing 32-bit kernels)
> >      * we might still make a bad choice here.
> >      */
> >
>
> I think this lowmem does not mean below 4GB. and it is to make sure
> the initrd_start > memblock_start_of_DRAM for Linux address range check.

The wording of this comment pre-dates 64-bit CPU support: it
is talking about the requirement in the 32-bit booting doc
https://www.kernel.org/doc/Documentation/arm/Booting
that says
"If an initramfs is in use then, as with the dtb, it must be placed in
a region of memory where the kernel decompressor will not overwrite it
while also with the region which will be covered by the kernel's
low-memory mapping."

So it does mean "below 4GB", because you can't boot a 32-bit kernel
if you don't put the kernel, initrd, etc below 4GB.

thanks
-- PMM