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

[Mike Rapoport]

On Mon, Mar 18, 2019 at 06:46:18PM +0530, Anup Patel wrote:
> 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??

That should be panic() rather than warning. If the trampoline_pg_dir will
map everything, it can be emitted during the initialization of
swapper_pg_dir.
 
> >
> > 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
> 

-- 
Sincerely yours,
Mike.