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 15:50, Schspa Shi <schspa@gmail.com> wrote:
>>
>>
>> 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:
>> >> > 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.
>
> A *32 bit* kernel cannot -- it is completely unable to access
> anything above the 4GB mark when the MMU is off, as it is on
> initial boot. This QEMU code handles both 32 bit and 64 bit
> kernel boot. These days of course there is 64-bit only hardware,
> and that might choose to put its RAM above the 4GB mark,
> because it isn't ever going to boot a 32-bit kernel anyway.
>

Yes, I think we should accept this patch, because it will not affect
32-bit devices, and provides support for 64-bit devices to put initrd
above 4GB.

> thanks
> -- PMM


-- 
BRs
Schspa Shi