Peter Maydell <peter.mayd...@linaro.org> writes:
> On Tue, 8 Nov 2022 at 15:50, Schspa Shi <sch...@gmail.com> wrote: >> >> >> Peter Maydell <peter.mayd...@linaro.org> writes: >> >> > On Tue, 8 Nov 2022 at 13:54, Peter Maydell <peter.mayd...@linaro.org> >> > wrote: >> >> >> >> On Tue, 8 Nov 2022 at 12:52, Schspa Shi <sch...@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
