Re: [PATCH] RISC-V: Fix Maximum Physical Memory 2GiB option for 64bit systems

[Mike Rapoport <rppt@linux.ibm.com>]

On Tue, Apr 02, 2019 at 03:00:02PM +0530, Anup Patel wrote:
> On Tue, Apr 2, 2019 at 2:05 PM Mike Rapoport <rppt@linux.ibm.com> wrote:
> >
> > On Tue, Apr 02, 2019 at 06:02:38AM +0000, Anup Patel wrote:
> > > The Maximum Physical Memory 2GiB option for 64bit systems is currently
> > > broken because kernel hangs at boot-time when this option is enabled
> > > and the underlying system has more than 2GiB memory.
> > >
> > > This issue can be easily reproduced on SiFive Unleashed board where
> > > we have 8GiB of memory.
> > >
> > > This patch fixes above issue by reserving unusable memory region in
> > > setup_bootmem().
> > >
> > > Signed-off-by: Anup Patel <anup.patel@wdc.com>
> > > ---
> > >  arch/riscv/mm/init.c | 8 ++++++++
> > >  1 file changed, 8 insertions(+)
> > >
> > > diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> > > index 5fd8c922e1c2..6b063f20a9d0 100644
> > > --- a/arch/riscv/mm/init.c
> > > +++ b/arch/riscv/mm/init.c
> > > @@ -121,6 +121,14 @@ void __init setup_bootmem(void)
> > >                        */
> > >                       memblock_reserve(reg->base, vmlinux_end - reg->base);
> > >                       mem_size = min(reg->size, (phys_addr_t)-PAGE_OFFSET);
> > > +
> > > +                     /*
> > > +                      * Reserve from the end of usable area to the end of
> > > +                      * region
> > > +                      */
> > > +                     if ((reg->base + mem_size) < end)
> > > +                             memblock_reserve(reg->base + mem_size,
> > > +                                              end - reg->base - mem_size);
> >
> > The memory above MAXPHYSMEM should not be reserved. It should be either
> > removed from memblock with memblock_remove or not added at the first place.
> >
> > Frankly, I fail to understand the logic behind setting PAGE_OFFSET to
> > MAXPHYSMEM and then using PAGE_OFFSET as the limit for accessible physical
> > memory. Still, as it is there, you can set MAX_MEMBLOCK_ADDR=PAGE_OFFSET in
> > arch/riscv/include/page.h and then early_init_dt_add_memory_arch() will
> > simply ignore the memory above PAGE_OFFSET.
> 
> Little explanation about current code ...
> 
> The current code, assumes PAGE_OFFSET to have value such that
> upper-bits are 1s lower bits are 0s.
> For example,
> 0xc0000000 (32bit),
> 0xffffffff80000000 (64bit), and
> 0xffffffe000000000 (64bit)
> 
> For above PAGE_OFFSET values, -PAGE_OFFSET is size of virtual
> address space and maximum supported physical memory is also
> -PAGE_OFFSET hence MAXPHYMEM is tied with -PAGE_OFFSET.

Right, but I what I don't understand is *why* PAGE_OFFSET is used as the
maximal supported physical address.
 
> If we try to force some arbitrary PAGE_OFFSET then things break.

Apparently they will :)
 
> Regards,
> Anup
> 

-- 
Sincerely yours,
Mike.