bug in identity map for 4KB pages?

[Stuart Yoder]

> -----Original Message-----
> From: Mark Rutland [mailto:mark.rutland at arm.com]
> Sent: Wednesday, July 29, 2015 6:43 AM
> To: Ard Biesheuvel; Yoder Stuart-B08248
> Cc: Marc Zyngier; Catalin Marinas; Newton Peter-RA3823; Will Deacon; linux-arm-kernel at lists.infradead.org
> Subject: Re: bug in identity map for 4KB pages?
> 
> On Wed, Jul 29, 2015 at 08:47:10AM +0100, Ard Biesheuvel wrote:
> > On 29 July 2015 at 04:37, Stuart Yoder <stuart.yoder at freescale.com> wrote:
> > > Our system has RAM at a high address, and previously required using 48-bit VA
> > > in order to have an idmap that covered all of RAM.
> > >
> > > In testing on 4.2-rc4, which now contains support for the increased VA range
> > > of the idmap without using 48-bit VA, I'm finding that things work for
> > > 64KB pages, but do not for 4KB pages.
> > >
> > > Is there any known limitation here with 4KB pages?  Any ideas?
> > >
> >
> > You probably have memory at 0x8000_0000 and at 0x80_8000_0000, right?

Yes.  We have 2GB of memory at 0x8000_0000 and the remainder at 
0x80_8000_0000.

> > So the physical arrangement still requires more than the 39 bits of
> > virtual address space you get with 3 levels, even if the ID map can
> > cope now. That is why you get the 0x40_0000_0000 virtual address:
> > __phys_to_virt() just wraps to a positive number.
> 
> Ah, I see. So it's the linear mapping rather than the idmap which is the
> problem.
> 
> We cut memory which would fall below the start of the linear map in
> early_init_dt_add_memory_arch, by cutting memory below phys_offset.
> 
> It looks like we already have the logic for cutting memory beyond the
> end of the linear map, so we should just need to override the limit.

What is the effect of cutting that memory?  ...are we losing something?

> Stuart, does the below patch prevent the panic you see?

I will try it and let you know...can try the suggested patch by
Ard as well.

Thanks,
Stuart