Re: [PATCH v2] arm64/efi: prefer AllocatePages() over efi_low_alloc() for vmlinux

[Will Deacon <will.deacon-5wv7dgnIgG8@public.gmane.org>]

On Tue, Jul 28, 2015 at 10:24:23PM +0100, Ard Biesheuvel wrote:
> On 28 July 2015 at 23:17, Matt Fleming <matt-mF/unelCI9GS6iBeEJttW/XRex20P6io@public.gmane.org> wrote:
> > On Fri, 24 Jul, at 01:38:27PM, Ard Biesheuvel wrote:
> >> When allocating memory for the kernel image, try the AllocatePages()
> >> boot service to obtain memory at the preferred offset of
> >> 'dram_base + TEXT_OFFSET', and only revert to efi_low_alloc() if that
> >> fails. This is the only way to allocate at the base of DRAM if DRAM
> >> starts at 0x0, since efi_low_alloc() refuses to allocate at 0x0.
> >>
> >> Tested-by: Haojian Zhuang <haojian.zhuang-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>
> >> Signed-off-by: Ard Biesheuvel <ard.biesheuvel-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>
> >> ---
> >> v2:
> >> - reshuffle code flow to make it more logical, and have only a single
> >>   memcpy() invocation at the end of the function
> >> ---
> >>  arch/arm64/kernel/efi-stub.c | 41 ++++++++++++++++++++++++++++++++---------
> >>  1 file changed, 32 insertions(+), 9 deletions(-)
> >
> > Would it be easier if we allow efi_low_alloc() to return 0x0 for some
> > uses? If you don't need the preference for low allocations, probably
> > not, but I don't want to see us working around limitations in
> > efi_low_alloc() instead of just fixing it.
> >
> 
> This workaround fixes another issue as well: the arm64 kernel needs to
> be loaded 512 KB above a 2MB aligned boundary, and using
> efi_low_alloc() as we do loses (2 MB - 512 KB) at the bottom if part
> of that 512 KB is occupied, since efi_low_alloc() is not aware of the
> fact that the first 512 KB will remain unused.
> 
> What would be most helpful is if efi_low_alloc() could take an offset
> param in addition to the alignment, i.e., alignment == 2MB and offset
> == 512 KB. The offset would default to 0, reverting to the original
> behavior.
> 
> If you'd be ok with such a change, I can propose it instead, and wire
> it up into this function.

I already merged the original patch, so if you propose anything extra,
please do it on top of that!

Will