Re: Exporting discontiguous System Memory to EFI STUB

[Laszlo Ersek <lersek@redhat.com>]

On 09/15/16 10:45, Sakar Arora wrote:
> Hi
> 
> This is in aarch64 UEFI context.
> 
> The efi stub code ignores any memory nodes in the device tree. It
> only relies on the UEFI memory map for memory info.
> 
> In such a scenario, how can one export discontiguous regions of
> system memory to the efi stub? There seems to be only one way to
> inform UEFI about system memory, via PcdSystemMemoryBase.
> 
> Looking at the latest Arm Juno code, it seems like building a memory
> resource descriptor hob, for the extra memory region, does the trick.
> Would that be the best way to go?
> 
> Suggestions please.

There are two ways.

First, in the PEI phase, you can produce memory resource descriptor HOBs
that will describe system memory areas. When the DXE core starts, it
will convert the suitable HOBs to EfiGcdMemoryTypeSystemMemory memory
space. During DXE and BDS, boot/runtime code/data allocations will be
satisfied from these. Then the UEFI memmap will reflect those
allocations, and the system memory left unused, to the EFI stub.

Second, you can add EfiGcdMemoryTypeSystemMemory memory space during and
after the DXE phase, explicitly, using the DXE services. (IIRC, the PI
spec says that memory space added this way may be picked by the UEFI
memory allocation system immediately; IOW, it may immediately become
available to the pool and page allocation boot serivces, to allocate
from. IIRC, in edk2 this actually happens.) The rest is the same as
above, wrt. the UEFI memmap.

You can see an example for the second method under
"ArmVirtPkg/HighMemDxe". I think it might be particularly close to your
use case, as it practically translates the memory nodes found in QEMU's
(copied) DTB to EfiGcdMemoryTypeSystemMemory memory space.

(Ard, when do you plan to port this driver to FDT_CLIENT_PROTOCOL? :))

Thanks
Laszlo