Re: [GIT PULL] x86/mm changes for v4.4

Matt Fleming
2015-11-07T10:03:28Z

On Sat, 07 Nov, at 08:05:54AM, Ingo Molnar wrote:
>
> * Matt Fleming <matt-mF/unelCI9GS6iBeEJttW/XRex20P6io@public.gmane.org> wrote:
>
> > On Thu, 05 Nov, at 01:33:10PM, Linus Torvalds wrote:
> > >
> > > And if this turns out to be due to EFI wanting those permissions, what should
> > > we do? People have talked about running the EFI callbacks in their own private
> > > page table setup, which sounds like the right idea, but until that actually
> > > *happens*....
> >
> > We have separate page tables today, for a few reasons, but mainly it's
> > so that we can have an identity mapping of memory present in the
> > region usually used by user processes - broken firmware still uses
> > those identity mappings even after the kernel tells it they're
> > invalid.
> >
> > Note that when I say "separate" I'm talking about trampoline_pgd[]
> > which is also used by the x86 suspend/resume code.
> >
> > However, turns out that the issue with the current scheme is the fact
> > that trampoline_pgd[] actually shares a couple of PGD entries with
> > swapper_pg_dir as can be seen in setup_real_mode(),
> >
> >
> > trampoline_pgd = (u64 *)__va(real_mode_header->trampoline_pgd);
> > trampoline_pgd[0] = init_level4_pgt[pgd_index(__PAGE_OFFSET)].pgd;
> > trampoline_pgd[511] = init_level4_pgt[511].pgd;
> >
> >
> > So when we map the EFI regions in efi_map_regions() we're inserting
> > them into swapper_pg_dir also, which is why you're seeing the
> > warnings.
> >
> > If I remember correctly the rationale for using trampoline_pgd[] was
> > that it already did what we wanted (provided the identity mapping) and
> > would save us the overhead of maintaining more page tables for no good
> > reason. Obviously this entire thread is a good reason.
> >
> > I suggest we stop using trampoline_pgd[] (since it has a good reason
> > for sharing the kernel mapping PGD entries) and create our own so that
> > we can isolate EFI completely.
>
> Ok. Could you please make this fix a priority for upcoming EFI changes?

Yep, I'll get on it.