Re: [PATCH 3/3] RISC-V: Allow booting kernel from any 4KB aligned address

[Anup Patel]

On Mon, Mar 18, 2019 at 12:48 PM Mike Rapoport <rppt@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxxx> wrote:
> > >
> > > On Fri, Mar 15, 2019 at 9:28 PM Mike Rapoport <rppt@xxxxxxxxxxxxx> 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