Messages in this thread | | | From | Anup Patel <> | Date | Mon, 18 Mar 2019 18:46:18 +0530 | Subject | Re: [PATCH 3/3] RISC-V: Allow booting kernel from any 4KB aligned address |
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On Mon, Mar 18, 2019 at 12:48 PM Mike Rapoport <rppt@linux.ibm.com> wrote: > > On Sat, Mar 16, 2019 at 04:55:30AM +0530, Anup Patel wrote: > > On Fri, Mar 15, 2019 at 9:52 PM Anup Patel <anup@brainfault.org> wrote: > > > > > > On Fri, Mar 15, 2019 at 9:28 PM Mike Rapoport <rppt@linux.ibm.com> wrote: > > > > > > > > I still don't get why it is that important to relax alignment of the kernel > > > > load address. Provided you can use the memory below the kernel, it really > > > > should not matter. > > > > > > Irrespective to constraint on kernel load address, we certainly need > > > to allow memory below kernel to be usable but that's a separate change. > > > > > > Currently, the memory below kernel is ignored by > > > early_init_dt_add_memory_arch() in > > > drivers/of/fdt.c > > > > > > > I explored the possibility of re-claiming memory below kernel but then > > we have an issue in this case. > > > > For RISC-V kernel, PAGE_OFFSET is mapped to kernel load address > > (i.e. load_pa in this code). The va_pa_offset is based on load_pa so linear > > conversion of VA-to-PA and PA-to-VA won't be possible on the memory > > below kernel. I guess this is why early_init_dt_add_memory_arch() is > > marking memory below kernel as reserved. Is there better way to do it?? > > > > We started exploring ways to re-claim memory below kernel because > > we are trying to get Linux working on Kendryte K210 board > > (https://kendryte.com/). This board has dual-core 64bit RISC-V but it > > only has 8MB RAM. > > Huh, 8MB of RAM is tough... > > It is possible to use the memory below the kernel, e.g x86-64 does that. > But it is definitely a separate change and with such RAM diet using 4K > pages seems unavoidable. > > I still have concern about using 4K pages whenever the load address is not > 2M (4M) aligned. People tend to not pay enough attention to such details > and they would load the kernel at an arbitrary address and get the > performance hit. > > I think the default should remain as is and the ability to map the kernel > with 4K pages (and use 4K aligned load address) should be a Kconfig option.
I agree people will tend to not pay attention on the load address alignment but this is also possible with current approach. Currently, if user boots kernel form any non-2M aligned address then we don't see any prints at all which let's users think it to be kernel bug. In fact, I have done same mistake couple of times.
Another approach (apart from kconfig option) would be to throw big-fat warning when users boot kernel form 4K aligned load address this way atleast kernel boots instead of no prints. Your thoughts??
> > Another thing I'd like to suggest is to completely split swapper_pg_dir > initialization from setup_vm() and keep this function solely for > initialization of the trampoline_pg_dir. The trampoline_pg_dir can use > large pages and the memory below the kernel start can be mapped there > simply by mapping the entire large page containing the kernel start. > Then, the swapper_pg_dir setup can run with virtual memory enabled and can > have much more flexibility.
Sure, this is a good suggestion. I will add this as separate patch in this series.
Regards, Anup
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