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

[Schspa Shi]

Peter Maydell <peter.maydell@linaro.org> writes:

> 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.
>

Thanks for your comment.

In this view, initrd shouldn't be placed higher than 4GB ? But it
seems the Linux kernel can boot when there is no memory below 4GB.

I know that lowmem is needed for SWIOTLB etc. It will be used to make
the 32bit IP work without IOMMU. But it seems it's not required to
boot.

> -- PMM

-- 
BRs
Schspa Shi