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

[Peter Maydell]

On Tue, 8 Nov 2022 at 13:54, Peter Maydell <peter.maydell@linaro.org> wrote:
>
> 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.

A kernel person corrects me on the meaning of "lowmem" here -- the
kernel means by it "within the first 768MB of RAM". There is also
an implicit requirement that everything be within the bottom 32-bits
of the physical address space.

-- PMM