On Wed, 16 Nov 2022 at 06:11, Schspa Shi <sch...@gmail.com> wrote: > > > 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.
Yes, I agree. However since it doesn't cause a problem for any of the machine models in upstream QEMU, I think we should leave it until after the in-progress 7.2 release, so that we have plenty of time to investigate just in case it does cause an unexpected issue on 32-bit boards. This patch is on my list to review and deal with when 7.2 goes out and development reopens for 8.0 (should be in about four weeks). thanks -- PMM
