Re: [PATCH v3 06/10] arm64: efi: add EFI stub

[Jeffrey Hugo]

On 2/8/2017 10:03 AM, Mark Rutland wrote:
> On Wed, Feb 08, 2017 at 10:35:02AM -0600, Timur Tabi wrote:
> > On 02/08/2017 10:29 AM, Ard Biesheuvel wrote:
> > > > > > > +       status = handle_cmdline_files(sys_table, image, cmdline_ptr,
> > > > > > > +                                     "initrd=", dram_base + SZ_512M,
> > > > > > > +                                     (unsigned long *)&initrd_addr,
> > > > > > > +                                     (unsigned long *)&initrd_size);
> > > > >
> > > > > So I know this patch is almost three years old, but why is there a
> > > > > 512M limit on the initrd size?
> > > How do you reckon this constitutes a limit?
> >
> > handle_cmdline_files() calls efi_high_alloc() with that limit.  I'm
> > still trying to understand all the details myself, but apparently
> > our firmware and initrd need to fit within the first 512MB because
> > of dram_base + SZ_512M. When we change "dram_base + SZ_512M" to
> > "~0", everything works.
>
> Just to check, how big is that initrd?
>
> I guess it's possible that there simply isn't sufficient contiguous free
> memory in that range, even if the initrd isn't that large. Can you share
> the EFI memory map dump from booting with efi=debug?
>
> We originally needed to restrict this to ensure that the kernel could
> map the initrd (and I think the 512M restriction specifically was
> inherited from the DTB mapping restriction). Since then, we have relaxed
> things in the kernel, and today Documentation/arm64/booting.txt says:
>
> 	If an initrd/initramfs is passed to the kernel at boot, it must
> 	reside entirely within a 1 GB aligned physical memory window of
> 	up to 32 GB in size that fully covers the kernel Image as well.
>
> ... so I think the EFI stub should be able to take advantage of that
> relaxation.

I agree.  The wrinkle I can see in this is it looks like KASLR can put 
the kernel anywhere in RAM.  How do we ensure initrd is within 32GB of 
the kernel on a system with 256 GB of RAM?

> Ard?
>
> Thanks,
> Mark.
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--
Jeffrey Hugo
Qualcomm Datacenter Technologies as an affiliate of Qualcomm 
Technologies, Inc.
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